#1
Floating round on youtube you can find some isolated just guitar/vocal/bass/drum parts of songs. Where do these come from?

Are they people with access to original unmixed tapes or can they be isolated with software?

e.g.
http://www.youtube.com/watch?v=JnMVWqGcPvo Foo fighters - guitar only -pretender
http://www.youtube.com/watch?v=Xm7w5al2n7o metallica - bass only - master of puppets
The only 6 words that can make you a better guitarist:

Learn theory
Practice better
Practice more
#2
You'll find that most single instrument tracks are on games like guitar hero and rockband that allow you to practice along to the track without that specific instrument in the song. As for how its done I don't particularly know. I've seen people attempt to do it just by cutting different frequencies out. Maybe they do have the master tracks, you never know...
Main Rig
ESP LTD MH-350NT
Korg Pitchblack > Digitech Bad Monkey
Peavey 6505+ 112 w/ 2x12 (Celestion Vintage 30 and WGS Veteran 30)
[Effects Loop] Boss GE-7 > Boss CE-5 > TC Electronic Flashback
#3
I know that at least some folks figured out how to rip the song files from Guitar Hero II, because I have an MP3 of Jordan that was gained in such a fashion.

Rock Band, at least, uses the original masters - Harmonix gets the actual song stems from the artist/record label and uses them to mix the game tracks.

As a result of these two things, I'm not surprised that the individual master tracks are floating around the 'net. Harmonix puts the stems on disc, people rip them off disc. That said, I don't know if other people acquire/create them differently.

It's also possible to just record your own version. In other words, some of the single-instrument tracks may have been recorded by someone other than the original artist. I, personally, have bass and drum tracks for Tommy Tutone's "Jenny", which I recorded myself; the bass I transcribed by ear and the drums were rendered from a MIDI track I made by following a drum tab I found on the interwebs.