find the instrumental, open audacity, load the original and the instrumental, zoom way the forget in till you can see the transients and stuff, and line up both tracks perfectly. invert the phase of the instrumental.
the better quality instrumental, the better quality acapella.
edit: after that you can load it up in a DAW or something and shove an equalizer on it. remove frequencies that are outside of the human speech range and just play with it till you get better quality. there may be a short frequency band instrument for example that slipped through the phasing.
cut the frequency band it occupies and it will delete the sound from the track, without (hopefully) loving with the vocals.