Where to get video footage for chroma key backgrounds free/cheap

Ideally HD widescreen but I'll take what I can get. I know I can get Istock photo credits and the like but it works out pretty expensive.Some I can film myself but I am also after flying, under water, skiing, outer space... that sort of thing. Is anyone aware of any sites that give it away for non profit?

Many of the pictures on Flickr are tagged with Creative Commons licenses, so if your use falls within CC you'd be able to use them. It may well be that there is CC video there too.

Thanks I'll do a search for cc stuff.