Has there been been a dedicated section here to trade or sell specimen book etc? I look on eBay and other places but I suspect that many here have copies they want to move on at different times, and still more who are looking for copies to acquire.
"type specimen" -(mineral) -(plant)
https://www.oakknoll.com (home of Oak Knoll Press, publisher, and Oak Knoll Books, rare and out-of print bookseller; online store is currently closed to the public for some unknown reason)
https://www.abe.com (worldwide selection, new and used books; owned by Amazon)
https://www.veatchs.com (everything about books, including specimen books)
Don't know, if they provide an English version of the website, or if they ship outside Germany & Austria. But you can use the German search term "Schriftmuster" or English "Type specimen" and get long listings. Then search the title, author, year via Google.
In any case you can start at Wikipedia at entries for a font or a famous type designer, foundry etc. Scroll down and look into the references.
Depending on your goal there are digitised versions on archive.org, collections, libraries, universities etc.
Also, the best specimen are the fonts themselves. Download free fonts. They are often low quality, but made with love by amateurs. The shapes of the letters are usable.
My aim is automatic identification of historic fonts to improve OCR results. I need them as font or scanned. Thus I prefer scanned specimen books. Scanning needs too much time. I have a collection of ~200 specimen books and ~3000 historic fonts.
Two of the best places to check out for type and typography books—and old books of all kinds—are the sites for the International League of Antiquarian Booksellers, ilab.org, and the Antiquarian Booksellers Association of America, abaa.org. Both have searchable listings for all of the members’ current holdings and may be searched by title, author, subject, pub. date, etc. And there are links to all of the member websites.
I still feel a TD market could work, nothing too fancy but similar to buy-sell Facebook groups just a place where members can post things for sale or thing they are looking for. All sales would be between the members as private individuals.
Sometimes I pay more for shipping than I do for the book!