K.
I have a subscription to www.emusic.com. For $10/month you get 30 downloads. They mostly focus on independent and alternative labels, so they won't have all of the mainstream popular music, though they do have a great selection and are constantly getting new stuff. All genres - classical, rap etc. They give you 50 free for signing up.
If you decide to sign up with them let me know first because I think I get some number of free downloads if I get listed as a reference.