Single songs are hard to come by. Most of the reliable old services are compromised or full of virus/spyware stuff. I only use torrents. Yeah, you're limited to full albums but sometimes if you search for a torrent of "best of" or "best metal songs" "best hip hop songs" you can get a list of a few hundred songs with just the better ones from many many artists. Try that. Utorrent is pretty decent program, small footprint. Usually find torrent links on pirate bay.
I actually buy used CD's off Amazon more than anything. They are like $1 with $3 shipping. Especially for bands I like.