Sorry for the lengthy post. There are some links below, but there are some incorrect statements here.
Clive said, “Will's right. You can't use existing recordings, but providing the composer has been dead for 75 years the music is in the public domain.”
No, not really. First, for works created on or after 1/1/78: copyright covers the live of the author (or in the case of a joint work - not “work for hire” - it’s the death of the last surviving member) +70 years, or if the work is copyrighted by a business entity or anonymous, it is 95 years from publication, or 120 years from creation whichever is shorter. For works created and published or registered before 1/1/78, copyright extends for 28 years for the first term with an optional renewal of 67 years (extended from 28 in 199

. (see
http://www.copyright.gov/circs/circ15a.pdf for more details).
Second, you can use anything you can get the rights to no matter when it was published. There are people who put works up for “free use,” but it’s rare and takes a lot of searching (see below).
Third, for a motion picture, there are 2 rights you have to get to use the work. 1: rights to the song. The composer holds the rights to the sheet music that the recording was made from. 2: master recording rights. The master recording of the music has a separate copyright. So, let’s say you want a Mozart song. The music is in the PD, any recording made before 1910 is in the PD, but the exact same song recorded published in 1963 is still covered under copyright.
archive.org is a great site, but some of the music is not free use. Just be careful and make sure to read everything.
http://flashkit.com/index.shtml has a lot of free use music and loops too, but not all are free so be careful.
Filmscheduling said “If you can befriend the music department of a university, the'll [sic] often have DAT archives of performances that they'll let you use.” Be very careful because the only way they have the authority to let you use this is if they own both the music rights (if applicable) AND the master recording rights. Students can get away with using copyrighted works for academic, non-commercial, work - technically they should still get it - and not get sued, but if the library doesn’t own the rights, they can’t grant you permission to use them. Finally, you should always get permissions in writing. That way, if the rights are contested, you can say that you thought you had acquired them in “good faith” that the person granting them had the right to do so, and then that person becomes liable for issuing permission. (please remember I am not a lawyer and this should not be taken as legal advice - check with a copyright attorney for the most accurate information)
Here are a couple, there are more sites out there. It just takes a few hours searching.
http://composerplanet.com/cgi-bin/wiki.cgi?FreeMusic
http://sonnyboo.com/music/music.htm
Search for “free use” and “public domain.” I’ve started collecting music on CDs fore personal use with a saved screenshot of the page that gives usage rights from the site I got it from because I’ve found that many of the sites I used to use have changed or disappeared, and that music is lost forever. That’s why I didn’t list more sites.
This site has good buyout music at reasonable prices. I have 2 of his cds for commercial and industrial use with a lifetime license for $40. He also posts a song a month for free.
http://www.zebramusic.com/lib.htm
Search for "buyout" or "royalty free."