Hey Teto,
Tell us a little about the premise of your film and what kind of music you want or what job it would do in the film. Music can be used a lot of different ways. A pop song could accompany a montage sequence, or big operatic orchestral music with taiko drums could underscore robots setting each other on fire...etc.
The role you need the music to play might determine the source you go to. As mentioned above, it can run the spectrum from emailing someone you come across (who has songs you like) on Myspace, to using tracks from music libraries, to using a friends band, or having someone write underscore cues specifically for your film. Could be a combination of all those approaches. There are varying scenarios of cost and/or permission. Like all other things, budget is a big factor.