Usually Jamando.
Searching for Create Commons songs with no NoDerivatives and no NonCommercial clause.
(Banners on the same page are the video are already commercial use.)
However it's hard to find songs that work well as a soundtrack.
No vocals (there are tons of songs tagged "instrumental" with someone suddenly starting to sing),
balanced stereo image that doesn't distract from the story (some artists seem to prefer strange ways of moving instruments around in space)
and not overly attention requiring (again to not distract from the story).