Creating A Capellas

I remember reading somewhere that there are methods for creating your own a capellas. Correct me if I'm wrong, but if you have both an instrumental as well as the original track, is it possible to somehow 'cancel' out the similarities of the instrumental with that of the original track? Thus, what you'd be left with would only be the vocal track. Are there other ways of doing this? Can someone explain or point me to a known resource? Thanks in advance! Lastly, does anyone have advice on searching for and obtaining a capellas; any good places to purchase them?

EDIT: My apologies, I wrote this up before I searched. There are a million threads on this topic; I'm not sure how to delete posts either. Disregard. I'll make sure to look before posting next time.