Amazon, iTunnes or I rip my own cd's. I have friends who own the cd then go out and buy the mp3, don't do that. Search the internet for an audio to mp3 ripper and voila. Now you can digitally back up and make your music library portable, for free.
Oh yes, I feel sorry for people when I find out they've been paying for the song twice. I've ripped most of my CDs to mp3s and it was very easy.