Yeah there are some tracks available at guitarbackingtrack.com. In my experience, guitarbt.com and guitarbackingtrack.com have pretty much the same tracks, but the selection on guitarbackingtrack is somewhat better. I've discovered five types of backing tracks, when looking for Dream Theater songs:
1. Midi tracks exported from Guitar Pro
2. User programmed midi tracks
3. User recorded tracks
4. 5.1 surround mix, guitar nearly isolated
5. Guitar Hero tracks, guitar isolated
6. The actual tracks from the original
The first two are the majority of tracks at least for my music, and they are simply horrible. I'd rather apply a nail gun to my ears than playing over those midi tracks. The user recorded are somewhat better but there are very few of them.
The 4th one works very well. On guitarbackingtrack they have some tracks from the Systematic Chaos album and some from the Live at Budokan DVD. The audio in both these are 5.1 surround mix. A while ago, there was a thread on guitarchaos.com where someone posted all of the songs from Systematic Chaos with the guitar tracks almost gone. Guitar Hero tracks would be even better, but I think they only have one Dream Theater song on GH...
The 6th only applies for the latest Dream Theater album, as the isolated tracks were included in the album's special edition.
Would be awesome if I could isolate those 5.1 tracks myself. Anyone done that before?
@Frozen
Thanks, I'm gonna take a look at that. I guess it's impossible to remove an instrument from a regular mixed song, but maybe adjusting the volume could tame the guitars making it better to play along the original.
Also, I saw a youtube movie where I guy used Audacity to remove the vocals from a song. Don't remember exactly what he did, but it was something about splitting the track into two stereo tracks and add an effect in Audacity called invert. It worked pretty well, but I don't understand how it works. Didn't say anything about muting other instruments, it only muted the vocals.