its almost impossible to obtain a studio acapella from a song. go to acapellas4u and blacklabelled.com for acapellas. blacklabelled also has nice instrumentals.
if you really wanna try, use audacity, split stereo track and invert it. watch how to do it on youtube.
my mashups ❶ http://soundcloud.com/mashed-potatoes
album download here ❷ http://www.mediafire.com/?1dzyy4q1qwb