I googled the following: library of congress baseball cards
This is the site:
http://memory.loc.gov/ammem/bbhtml/bbhome.html
I'm sure there are many major league cards but I was interested in the minors, so I clicked "About the Collection" and then "minors". There were 157 minor league players of different series; certainly not complete---I was kind of hoping for more.
Max was correct in stating this is where the ebay seller(s) get the images for their posters because as far as I can tell the only ones they sell are all available at LOC.