Another site:
www.audiogalaxy.com
Haven't been to this site in a year or so....it's probably changed since I was last there but, used to be a good site. Also, I do like the one that slease mentions...BuyMusic. This is the one I've been going to lately.
And, Napster is due back at some point this year although I never really was a fan of Napster. Will have to see how it changes.