Querty, iTunes is the obvious one, as much because it’s so easy to download tracks from, and if you use Shazam, it generally, but not always, gives you a direct link into iTunes to buy the track. The exceptions being tracks that either aren’t released yet, or just aren’t available.
Amazon have a good range of MP3 tracks, which you can download into iTunes, Bandcamp and Soundcloud also frequently have tracks that can be downloaded, either free or paid for. I get mine from all over the place, but iTunes ones are the easiest, others have to be downloaded onto your computer, then dragged into iTunes. Hardly a chore, really.
One thing I’d say, and that is to set your Touch, should you get one, to manually manage music and video; that way you’re always in control of what’s on your device, and you can just drag tracks into the device when you want to.