Have you tried torrents? I know extratorrent.com has tons of music and is very reliable and safe.
Also albumwash.com is great for entire albums.
You could also use MP3Torpedo it is basically a clone of the old limewire on the gnutella networks if that is something you may be interested in, but sometimes you have to dig through alot of trash and fake songs til you find the one you were looking for.
http://www.mp3torpedo.com/
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