In a word, yes.
     The things I'm going to discuss are "supplemental" only to one whose primary business has been, for decades, coins and paper money.  To most Americans, for example, it is coins that are the supplemental financial instruments, with checks and credit cards being the normal way they transfer funds, and to the issuers of stocks and bonds their certificates are far more significant than mere "petty" cash.
     From a collector's point of view there are a number of categories of obtainable financial relics.  Most are made of paper, and most date from the 19th and 20th centuries, though one occasionally finds items older than that.  Since the essence of collection is categorization, and since this is a relatively new field, I feel myself free to set up the categories as I please.  So here goes.

      A. Government special notes
      B. Checks
      C. Pay warrants
      D. Bills of exchange
      E. Money orders, travellers' checks
      F. Credit cards
      G. Debit cards
      H. Good fors
      A. Bonds
      B. Certificates of deposit
      A. Official papers (Indian fiscals, for example)
      B. Bank papers (statements, transfers, etc.)
      C. Private papers (ledgers, statements, correspondance, receipts, etc.)
      D. Legal papers (probates, partnerships, deeds, etc.)

 I think that's plenty of stuff to talk about.  Now for a more detailed discussion.

     A. Government special notes - these are in-lieu-of-cash notes issued for various reasons by governments.  Examples include the foreign exchange certificates of the defunct Socialist governments and the "food stamps" of the USA.  There are a lot of little issues, poorly researched for the most part, and lacking a catalog.  Some of the Socialist stuff is fairly easy to obtain, but a lot isn't.  USA food stamps are a tough field to try to collect.  I've written a couple of articles on some of the USSR series.
     B. Checks - documents with which the writer directs someone to give money on account to a third person.  Where a time of redemption is stipulated the papers are called "promissory notes."  Without a time stipulation the items are called checks or sight drafts.  Such documents are known, made of clay, from Babylonian times, c. 1500 BC.  I have found reasons to doubt every specimen I've seen offered for sale in the numismatic market.  Just call me ornery.  From Babylon there is a long hiatus in documentation, and then one begins to find, in places like medieval Italy, letters along the line of "Tomaso, you owe me 38 ducati. Please, when next you go to Siena, give 13 ducati to Alfredo Spinelli." I don't mean "find" in the sense that such documents are available for purchase by collectors, but rather that examples of such things are rather plentiful in the municipal libraries of Italian towns, and can be looked at if you are there.
     The earliest European banknotes are really checks drawn on a bank, their denominations entered by hand, and signed by a bank officer.  Such things as the 17th century Bank of England notes are theoretically available, but in practice are hardly ever offered.  Use of sight drafts and promissory notes grew in the 18th and early 19th centuries in England (and Holland?), though again, such documents are quite rare.
     Bank checks have since become general use items throughout the world.  You can bring cash to virtually any bank and turn it into a "window" or "cashier's" or "certified" or "bank" check.  Collecting such bank instruments is another matter, because where does a check go when it's been cancelled?  Back to the issuer, of course, which in the case of these checks is the bank, which likes to keep them on file.  So, for that reason, they're hard to get.
     Around the middle of the 19th century preprinted personal checks began to become popular in England and the USA, and by the 1860s their use became widespread.  Personal checks from that period on are fairly easy to come by, and are extremely cheap compared with contemporary banknotes.  Some, though by no means all, have nice artistic vignettes, and many of them carry documentary tax stamps.  Unfortunately, a lot of them were cancelled by cuts, punches, or even tears.  Checks of this period were often turned ad hoc into promissory notes, much as we will occasionally post-date checks today.
     Nowadays, use of personal checks is big in USA, Canada, UK, and perhaps a few other countries in the world, but they are not a global phenomenon.  Compared with bank checks they are relatively easy to collect, because, having been cancelled, they are usually returned to the writer, from whom we can entertain the possibility of acquiring them.  The average check has a low collector value, which makes people more inclined to throw them away than to sell them, and many people will not want to give up their cancelled checks for fear of someone attempting shenanigans with their bank account.
     There is at least one check collector's club that I know of, but I have not yet located it.  Laziness, or priorities, take your pick.  Someone will perhaps do my work for me and send me the address of this group.  I hope they do soon, for I am pretty much out of (modern USA) checks to sell to my clients.
     Some people collect checks issued by certain companies (e.g. Coca Cola), or certain people (e.g. famous ones), or certain locations.  One of my clients collects 1ยข checks, which, as you can imagine, is not easy.
 C. Pay warrants - basically the same as checks, but issued by a government agency.  The agency could be anything - municipal, school board, county, state, federal, military, etc.  These are harder to collect than personal checks for the same reason as the abovementioned bank instruments: the agencies tend to live a long time, longer than mere human people, and they rarely throw anything out.  By the same token, they are typically more popular, by and large, than personal checks.
 D. Bills of exchange - these are circulating checks.  The original purchaser buys one at the bank, and spends it by endorsing it to the next person, who repeats the process, and the thing can change hands dozens of times before finally being presented at the bank for redemption.  Such things are sometimes put into use when the currency is shaky, as in Germany and the USSR of the 1920s, and the well known (to some) hundis of India are another version of this type of instrument. Of the many types of hundis only a few are at all common, and the German and Soviet items I've seen are really pretty scarce.  Most were redeemed.
 E. Money orders, travellers' checks - these are cash substitutes, more secure than paper money.  One will occasionally find them for sale as collectibles, but not very often, because when they are cashed they are returned to the issuing agency.  Still, they can occasionally be found, in used, blank-unsigned, or specimen form.  Each type requires knowing someone on the inside, and one can assume that the insider contravened a regulation in removing them.  If the banks wanted you to have them they'd be all over the place, eh?
 F. Credit cards - the first of these were made in the 1950s and their use is now worldwide.  I use the term here in its broad sense to include bank cards, check guarantee cards, credit company cards (Visa, etc.), in-store cards (e.g. of gasoline companies), etc.   Technically these things remain the property of the issuer, and are supposed to be destroyed or returned when their use is done.  They are usually destroyed each year when the new one comes, and are hardly ever returned.  Enough of them filter out into the collector world that one can reasonably contemplate collecting them.
 G. Debit cards - are prepaid plastic cards that you can spend on a limited, but expanding range of items.  The most common varieties worldwide are the telephone cards.  These are very popular in about 50 countries, coming on strong in another 80, but have yet to catch on here in USA.  Generically, they are easy to come by, because they are sold where they are used, and are frequently discarded when they're used up. Due to the fact that most countries issue a continually changing roster of types and values, and since most issuers accept advertising and issue commemorative cards, there are already, in less than 10 years, tens of thousands of types.  Collecting them is popular in places other than here, and interest has approached boom levels in some countries.  I've written an article about plastic money.
 H. Good fors - there are several categories of these.  Sometimes municipal fair committees will put out good for notes, wooden nickels, etc. that will be redeemable locally during the town's centennial celebration or whatever.  There are also store cards: "Bring this in for a free ______, value ______..."  And finally there are the "manufacturer's coupons," which have a cash redemption value listed on them, so technically are a kind of money.  Some people collect the centennial money, but I've never met anyone who collects modern store cards or manufacturer's coupons.
 A. Bonds - theoretically well known items, in actuality most people have never seen them.  Though they are issued in great profusion by all sorts of private companies and government agencies, they are, for the most part, hard to find, the vast majority having been redeemed at maturity.  There are still plenty left to collect, however.  The available specimens belong to the following categories:
 1. issued by a defunct body
 2. unissued remainder released through sale of assets of  same
 3. illegally removed from the files of a "live" agency
 4. specimen
     The most commonly purchased American bonds are US Savings Bonds. These remain alive until redeemed, and I know of no one who has them for sale to collectors.
     There are also postal savings certificates of many countries, which are subject to the same restricted availability as larger bonds, but have the advantage to the collector of having lower values which makes them more likely to be forgotten in the bottom of a drawer for 50 years.
  I've written a short article about bonds and shares.
 B. Certificates of deposit - once again, you're supposed to surrender the paper when you redeem or update the deposit, so there are not too many around.  The few that are available fall into one of the abovementioned categories noted for bonds.
     These are general purpose partnership papers.  While theoretically subject to the same restrictions as bonds, in practice their circulation is considerably less restricted.  New companies are constantly coming into being, issuing shares, becoming defunct, etc.  That's the business cycle.  Share certificates become available upon the demise of the company.  Cancelled certificates for "live" companies are sometimes kept on file, not at the company office but at the office of the stock broker, who, going out of business or for some other reason releasing old files, will sometimes make such papers available.  Thus, for example, General Motors certificates of certain periods are not uncommon.
     This is my "everything else" category.  When you buy an old suitcase full of papers maybe you'll find a couple of shares or bonds, maybe a piece of currency, possibly a wad of old checks.  Most of what you find, however, will be this stuff.
 A. Official papers - in most countries your legal business has to be done on official forms, and in many countries you have to buy the paper from the government.  Sometimes the government charges an arm and a leg (e.g. Indian 500 rupee tax documents).  In USA until relatively recently certain classes of documents required a documentary tax stamp to be purchased and affixed.  There is no catalog of these things in general, though there is one on the fiscal papers of India.
 B. Bank papers (statements, transfers, receipts, etc.)
 C. Private papers (ledgers, statements, correspondance,  receipts, etc.)
 D. Legal papers (probates, partnerships, deeds, etc.)
     There is no organized collecting or dealing in these.  Every time someone dies a pile of these items is left for the executors to deal with.  Usually the convertible items are taken out and redeemed, the ledgers and accounts closed, the legal papers dealt with, then most of the stuff is thrown out and the rest is kept in the survivors' files. Only every now and then does some of it slip through the family's fingers and get out into the outside world to become an anonymous collectible.
     Three rules govern the value of these documents:
     1. Age - the older the better
     2. Fame - well known people carry a premium
     3. Collector "need" - about which you just can't figure.  If there's overt research potential, for instance, or if the collector is looking for certain names or places and you the dealer just happens to find them, well, lucky you.
     We're dealing with paper mostly, some of it old, some of it large. It is axiomatic when dealing with old papers that they should be preserved flat, but this can begin to take up a lot of room if you are building a large collection.  I've found postal mint sheet files to be useful for USA share certs, but Indian fiscals, for example, are too tall.  A typical solution is a wide file and large size polyethylene bags.  Valuable specimens can be matted and framed.  There are no stock manufactured display vehicles for large size paper.  You need to figure this angle before you get in too deep.

Bob Reis
POB 26303
Raleigh NC 27611USA
phone: (919) 787-0881
(8:30AM-10:30PM EST only please)
fax: (888) 503-8308