Not that public domain status has ever stopped visualists from using footage in sets, but if you want to mix in brilliant vintage video footage and be legal (perfect for that six hour set you’re doing at the FBI Intellectual Property Unit after-party in Vegas), here’s yet another answer:
Unlike the awesome treasure troves of the Prelinger Archives, there aren’t direct downloads. Fortunately, some of these movies are so obscure / awful / not popular with the kids today, they’re quite cheap on Amazon, and OpenFlix is more than happy to take your affiliate dollars when you buy them. Outdo Mystery Science Theater and mix in Santa Claus Versus the Martians this summer for a little Christmas in July. Or, marvel at the number of classic flicks (even multi-Academy winner A Farewell to Arms) are in the public domain. Better yet, a lot of the questionable copyrighted content is in the sound score. VJs, start your engines.