Music Minus One professional orchestration and musical accompaniment recordings.

The legendary MMO recordings are not just for piano concertos any more. Practice with a real symphony, band or other ensemble in studio recordings. You can even license these recordings to cut an album to.

Emusic  MP3  Downloads

An MP3 download subscription site where a monthly fee allows you to download mp3s. (They, and Rhapsody below, require a credit card for the free trial to prevent unethical people from having endless free trials.)

Real Rhapsody Unlimited Music Subscription

I personally use and swear by this service. I can find any song I need, and listen at high quality. Try the two week free test.