I have over a hundred songs downloaded from this site and I love it! What is really cool about it is that after purchasing the songs you can re-download as many times as you want to with different mixes...With or without guitar, with or without vocals, etc. I don't think the price is too bad...$3 per song avg.
Jammit is another site that has some unique features for drumless tracks, too.
I was also able to search enough and found somebody has gone through the Guitar Hero games and uploaded all the tracks from the game as a drumless track as well.
Good luck!
Happy Drumming!