Here are just a few links to some popular free stock video websites. This first one Stock footage for free you do need to sign up for but the footage is worth it.
Free stock footage you have to have quicktime but has some good stuff.
Artbeats gives away a free weekly clip just for signing up:
Of course you could just do a google search for public domain usage – but there is a lot of free crap out there and these sites are generally less crap a bit higher quality.