You could try archive.org , they have a pretty good collection of public domain music, video etc.

Other than that I would search for unsigned indie artists to find something you like then contact them directly. most would be more than willing to allow you to use it for small fee or even free.

Many of the major photo libraries also offer music files. Getty certainly does, although if their image rates are anything to go by, it'll be expensive to use them.

Syzygy

There are tons of them out there.

Look for music released under a Creative Commons license. There are several websites that offer streaming and downloads of CC licensed music.

Not all of it is licensed for commercial use, but much of it is.

Apart from my day job, I am a performer in an independent band, and we always love it when someone approaches us to use our music for a commercial, video or movie soundtrack. I agree with coalman on this...contact an unsigned band to use their music; oftentimes you will get a better product directly from the band (instead of the compressed mp3), and the possibility of receiving new/unreleased material. It's also a good way to form a long-standing relationship with people who create music; we are open to discussing projects that are not our usual style. Musicians want their music to be heard (and hopefully appreciated)...most will be willing to work with you.