Both these sites track a variety of MP3 blogs. If a post contains MP3 links, each adds those links to its searchable database. In other words, if you are looking for a song, try searching for it on The Hype Machine and elbo.ws. If it’s floating out there, you can find it in here.
For practice check out “Young Folks” by Peter Bjorn and John. I just searched for it on The Hype Machine and here it is.