The only officially released acapella is the one of Baby's Got A Temper. You can make some acapellas yourself if you have both the normal version with lyrics as an instrumental of the same track. How to do that, I think that Ekko, Zed or someone else on the Equipment board can help you out better.
actually, i think there is the acapella on LP (dunno what version for sure) of BGAT. but i think it's useless for remix ;) and yes, an acapella can be extracted with using the original and the intrumental version. but it's can be pretty complicated to do this (if versions were mixed with different equalizer sets for example). technically it looks like:
a = intrumental
b = original
c = vocal
a + c = b
b - a = c
b - c = a
but we need b-a formula. it mean you need take original and negative instrumental version, then mix it 50\50 or 100/100, but you need take totally the same parts to mix, i.e. the same position (it's pretty hard to find that position) in each parts. anyway, the main thing here is to understand the principle.
or just find acappela somewhere in the net...
btw, that technology allows to extract other samples from the track... but it's my own secret :)