I've been asked a few times by people interested in sound if I knew where they could get tracks to practice mixing with...and ususally not been able to give an answer.
However I've recently found THIS SITE which lets you download all the constituent parts for several known songs then see what you can do in terms of remixing them. I'm in the process of playing with Peter Gabriel's "Shock the Monkey" but there are a few other tracks as well.
You have to register with them to get to the downloads but this is quick and free.
Have fun!
Bob