Vitaminman has got it right on the money regarding the extraction of frequencies utilizing a wave editor as a solution.
Rather than seek the masters, the best thing to do is to find the song online a capella (just the vocal track), or to go to your favorite record store (focus on RECORD store), where they sell records to Dj's, and buy a record or CD single that has the song you want... usually the singles for Dj's have an a capella version as their fourth track (track one is the album cut, track two usually the radio edit, and track three is the instrumental... you get the idea.)
DJ's need a capella tracks to mix over different beats when they're in the club, plus they do a lot of interesting stuff and creative stuff when they make Tupac rap over a Timbaland beat, or some weird shit like that.
BTW "a capella" essentially means "vocal track," if you didn't know.
Good luck.