I use this site a lot when I can't find an MP3 I'm looking for online (since most everything can be found on YouTube). Check it.

As long as you have the YouTube link for whatever it is that you want an MP3 of, it'll work. It's amazing. I mostly use it for when there's a live version of something that I love that is literally impossible to find anywhere other than streaming on YouTube. Don't tell too many people about it.