Music in public domain can be found at http://www.pdinfo.com
Published and pre-recorded music–obtaining rights: Begin your search by calling the Harry Fox Agency in New York, (212) 370-5330. Many publishers, even if you contact them directly, will steer you to the Fox agency. If Fox does not represent the publisher you are seeking try calling the index departments of BMI, (212) 586-2000, or ASCAP, (212) 621-6160. Neither of these agencies clears rights but they may be able to provide information about whom to contact. You may also contact two smaller agencies that clear music rights, SESAC, (615) 320-0055 and AMRA, (813) 488-9695. In recent years, more publishers have been represented by Copyright Management, (615) 327-1517, and Bug Music, (213) 466-4352.
Some of the music information above is from an ITVS Prodution booklet. A link on their website leads to an order form for the production manual and accompanying forms. ITVS can be found at http://www.tvs.org/ .