Acapellas, at least as far as I've been able to find, are difficult to locate, especially for songs that weren't released as singles. Over the last few years I have found some sites that do offer acapellas, but these are usually FTP sites and don't stay around for very long.
http://www.acidplanet.com/contests/
This is ACID's site. On a monthly basis ACID is running remix contests where they upload sample packs, including the vocal tracks and some instrumental tracks, for people to remix. These are essentially acpacellas but usually broken down into multiple tracks; verse, bridge, hook for example. These are good but obviously the selection will be limited.
www.samplearena.com
This is just a start point for finding samples, acapellas, loops and soundfonts. Go to the section labeled SITES and you'll find over a hundred sites for downloading. Check all these sites and you may also find some other site offering acapellas as well, let me know if you do.