On the Web you can find sites that provide music for free download, but that doesn't mean permission has been given to use the music in your movie. Most of these sites provide the music for personal listening, but are still protected under copyright law from making "derivative works."
However, there are also sites that provide some free material and grant use of it for events such as iMovieFest. The way to find out what you can do with the music from a Web site is to look for a page called "Terms of Use." Often the permissions the authors have granted you will be spelled out there.
If you use music from a site that grants permission on a "Terms of Use" page, crediting the site in your movie is sufficient documentation that you have their permission.
To get you started, here are a few sites that we have run into which provide either licensed music or some kind of connection to the artist so you can email them and ask for permission. Remember to check the terms of use or license page to see what you can do with your downloaded music.
iMovieFest is produced by the student staff of Residential Technology Services