When stamps are supplied to post offices they are usually shrink wrapped in blocks the size of the sheet. The cover of the shrink wrapping usually has a picture of the stamps and the number of stamps in the package. Depending on the size of the post office the shrink wrapped package can contain 20, 50, 200, 2000 stamps.
A number of collectors get these given to them from local post offices although you can also buy them from the Philatelic Agency in Kansas City.
Do a search on e-bay using "deck cards" to see single examples. I collected these in the early 2010s and now am ready to dispose of them.