An downloadable instrumental website would almost certainly be a copyright violation site, unless the artist has released the track to the public. Peter Gabriel released one of his songs in Garageband format IIRC.
Removing vocals partially can be done in a very crude way with a combination of centre-cancelling and filtering, but the effects on the rest of the music ranges from audible to devastating. There are some software programs that will attempt it. Results vary.
Extracting an acapella from a stereo track is next to impossible. This is like asking to get the pork chop out of the sausage.
The best you can do in either situation above is to find a section of the song that has the instruments only (or the vocals only which is more rare) and sample just those sections, then loop those.
The other thing you can do is to get MIDI tracks of the instrumental tracks, and 'play' the track with MIDI instruments (software or hardware), or look for Karaoke disks with licensed versions of the tunes.