I’ve just stumbled across what looks to be a terrific archive of music that looks to be entirely usable. It’s termed ‘Royalty Free’ which is usually a sign that it isn’t (see this link for an explanation), however in this case it really is free. It’s all under Creative Commons Attribution licenses, the same ones we use. It’s great stuff, too.
You can browse by genre, or search by style/feel/tone — then you can preview the tracks before you download them and put them in your movies. Most of the tracks I’ve previewed have more than long enough for a SciCast film, which saves trying to loop them. You can try editing them so they finish neatly, or simply add a fade at the end of your film.
The composer/performer is Kevin MacLeod; if you use his work, remember to credit him in your film, and make sure you keep a note of which track you’ve used (so you can tell us, and we can link to it from the SciCast site).