1) search for them here:

http://lcweb2.loc.gov/pp/wpaposquery.html

2) download

3) print!

Unfortunately, the high-res versions aren't always THAT high-res, but most are pretty good with a bit of photoshop tweaking.

Looks like they're constantly updating it with more and more posters as well...really like this one:

http://memory.loc.gov/service/pnp/cph/3f00000/3f05000/3f05400/3f05439r.jpg

JLM - did anything come of this? i am also very interested in reproductions of these posters.