I hope this isn't a dupe. You can download 14 retro style space exploration posters for free from the Jet Propulsion Labs/NASA:
Visions of the Future
There are also 903 WPA posters produced from 1936 to 1943 at the Library of Congress:
Search Results: "" - Prints & Photographs Online Catalog (Library of Congress)
Visions of the Future
There are also 903 WPA posters produced from 1936 to 1943 at the Library of Congress:
Search Results: "" - Prints & Photographs Online Catalog (Library of Congress)