Elliot Jay Stocks asked the following on Twitter just now:
Looking for CC-licensed retro (or pseudo-retro) sci-fi illustrations... anyone got any pointers?
Not only am I interested in answers to Elliot's question, but I also thought it might be useful to have a permanent archive of answers here on doctype.
Use wikimedia commons to search for sci-fi images. Wikimedia commons is a repository of millions of cc licensed (and many completely free ones too) images and display licencse details in full for each image. Usually have multiple resolutions, descriptions, etc...
Here's the sci-fi category - looks pretty awesome :D