>>15There are quite a few ways to do this kind of thing, depending on the materials you have available, and your skill level
I like to first look acapellas at
http://www.acapellas4u.co.uk/http://producer-area.pl/If its not there I'd do it myself
Alot of music has the vocals placed in the centre, and instruments placed around in=t in the stereo field. Because of this, you can use various tools and methods to isolate vocals.
1. Sample Inversion
Simply get a high quality version of both the instrumental of the track you want the acapella from, and the original. Then drop them both into your DAW (cubase, fl, pro tools, audacity, doesnt matter) Invert the waveform of one (usually in the audio settings), and then line them up.
The theory behind it is that since the waveform was inverted, it will negate the other. I.e. The instrumental with negate the instrumental portion of the full track, leaving only the vocals. This method is not perfect, and only works in a few cases, but seriously give it a try. You can even use bits from the original as instrumentals if an Instrumental mix doesnt exist. If a riff is playing on its own, and that very same riff plays under a vocal in a different section, then it is possible to negate it that way.
I'll post other methods later on, gotta write it all out.