LegalSounds
It is obviously not nearly as smooth as itunes, but it sure as hell gets the job done. The major drawback is that you have to deposit money ahead of time. But depending on how much you deposit, they will give you a ton of free downloads. I usually always put in $50 at a time and I get 80 free downloads with it ($7.20 worth)
So, basically with a $50 deposit you get to download 635 songs. I'm sure you math wizards realize that on itunes that would cost you at least $635 dollars. Pretty awesome if you ask me.
I was reading up on the legality of it and apparently they have yet to pass any laws against use of foreign websites for music downloads. Which means I get to tell myself that I am doing nothing wrong.
The site has pretty much anything you could possibly want. On the few occasions that they don't have what I want, they have an album suggestion area that you can use. Within a week or two they usually upload the said album/songs for download. They also let you know via email that they recently got the album you asked for.