How can I get instrumentals and acappellas from songs?

I want to do some mashups Mylo style if anyone's familiar. Anyways, I need to get the instrumentals seperate from the singing/voices so I could use say the instrumental from "Tiny Dancer" with a Notorious BIG acappella so that Biggie is singing over the Elton John instrumental (this was done already on an album). Is there some way I can split up the vocals from the music or at least come close with some software or something or do I need to actually have the instrumentals/acappellas? Thanks.