http://www.beatport.com
I'm promoting these guys because i seriously like this store. If you like dance music, this is the place to buy from. They have a ton of hard to find artists, underground music, etc. The site design also flows really well, and you get nice long previews of the music, which other sites only give ya like 30second to 1minute previews.
In a sence, its an amazing store, check them out.
Review: 5/5 stars.
Just a word of caution if you have never downloaded purchased music before:
1. Read instuctions carefully.
2. It can be addictive, so pace yourself with it. I'm already addicted to buying music from the Nu-Breaks section in particular lol.