Hi, Strider - I have recently discovered poster stamps myself, and have been learning as much about them as fast as I can. Let me share a little of what I've gleaned from my recent "discovery".
I've been collecting stamps for 55 of my 58 years, and I had never even HEARD of "poster stamps" until about a year ago. Last year I was seeking various foreign revenue catalogues to build up my personal library, and I would notice that when I would search for foreign ("foreign", to me, is anything outside the US - I live in New Orleans, LA) revenue stamps... I would often see a listing or a link to a "poster stamp". Finally after seeing this mentioned a few times, I decided to investigate and good lord, it's a new money pit for me. But I couldn't be happier, because these are absolutely amazing,
First, there are the exhibition stamps (German: Ausstellung) that you mentioned in your original post. These are really nice and my personal favorites - whether it is an exhibition for paper products, electrical items, dogs, aircraft, or even a garden show - what makes these so easy for me to collect is that they are DATED. Because I came from collecting postage stamps, I am used to the chronological order associated with collecting, and that made collecting these quite comfortable for me. There is a classic catalogue, if you can find it: Cazin & Rochas published, in 1914, a catalogue titled "Catalogue des Timbres Commémoratifs" which is a very good (not complete, but excellent nonetheless) listing of exhibition poster stamps. The catalogue reaches back to what may be the earliest poster stamp from the 1840s, through 1914. German cities from 1907-1914 have the most listings. The catalogue lists the stamps (description only, no photographs) and color/perf varieties through summer 1914. (WWI apparently brought poster stamp production to a near halt) I bought a copy from a German bookseller last year for about $80 and it was worth it. It's in French, which I can read but is also easy to understand if you use google translate on the camera setting. The book is very difficult to find but really nice to have.
Next are product advertisements. Some of these are just stunning, with some of the best designers (Ludwig Hohlwein, Kurt Bottcher, etc.) behind many of these. Charles Kiddle's catalogues, if you can find them, are an excellent source of information. There are other works out there as well, look for the bibliography on the poster stamp collectors club website http://www.posterstampcc.org
A third type of cinderellas that I like are the World War I-related items. There are the famous "Delandre" stamps, which are mostly French military but also include a lot of propaganda stamps as well - some 4,000+ were produced between 1915-1917 when the proprietor was arrested for various crimes. Besides the Delandre stamps, there are various propaganda items produced by the Central Powers. Some of these are foil labels, some shaped like bombs or mortar shells, with messages like "Gott strafe England" ("God punish England"), or other wartime-related messages. There are many colors/varieties of these as well. These are catalogued in a 1993 book by New Zealander Alan Jackson titled "For God, Kaiser, and Fatherland". There are many other stamps listed in here, propaganda labels, charity stamps, etc - it's fascinating area all in itself. And there are WAY more of these than I ever imagined existed.
These are the three primary types of cinderellas that I collect. There are some really beautiful charity seals out there as well, and then there's the fascinating ephemera that Jim Drummond had been capturing in his works.
But since you're focusing on the exhibition stamps, you may want to learn a few of the designers who were behind many of those that were produced. I know there is a beautifully illustrated set of books - in German - that lists many of the major works of many graphic artists/poster designers, but it is expensive (5 vols for about $250 if you can find them). That's if you're really really interested in the artwork itself on the stamps.
Sorry for being so long-winded! But this is a new subject to me as well and is really fascinating. Look at listings on eBay
, do google image searches on topics and artists, see what you like. No need to get everything - just get the ones you think look nice. That's what I'm doing - for now, at least!