There are loads of places you can download music - it just depends what you want to do - pay per track (which is what I do) or pay a monthly subscription and download as many tracks as you want.

I'll try to remember to post a note at KK about the fic, although its a Studio 60 one (my second!) and I'm not sure if everyone there is into that, so... (but I do have a JD one on the go, too!)