How available are individual stems for music in general?

[changing_seasons]

Let me just say, I absolutely LOVE stems. Hearing the individual instruments for the music I like gives the listening experience another dimension entirely. Hearing Jordan's arpeggiated orchestra runs after the guitar solo in TCOT by themselves always gives me chills.

I wonder if anyone here knows a bit about how this stuff works in the music industry. I assume the artist or maybe the producer keeps all the individual tracks stored after an album is finished. I know jammit.com have a lot of stems from older music, so I'm guessing there is both a marked and some availability for this.

For progressive music in particular, a lot of fans are musicians, so I think it would make great sense to make individual tracks available. Now Dream Theater are pretty well covered on jammit (up until SFAM anyway), but I know I would pay more than I'm willing to admit for individual tracks to Neal Morse's albums.