There are numerous sites for backtracks (like http://www.guitarbackingtrack.com and http://guitarbt.com), i.e. songs with no guitar tracks that allow you to jam over them yourself or make a cover of a known song. The best backtracks are originals with all other tracks, even vocals. These are quite hard to come by as no artists (at least that I know) release such tracks officially. They are slightly more common now though compared to a couple of years ago, as they can be made by ripping the individual tracks from Guitar Hero or Rock Band. Probably not quite OK from a copyright perspective, but as long as you don’t make any money from them I figure it’s morally fine (record labels: I’d be happy to pay for backtracks like these! Please sell them!).
The worst kind of backtrack is a midi file converted to an audio track. These usually sound extremely cheap, but are quite common.
If you can’t find a good backtrack (with or without vocals) you can make your own from a Guitar Pro file. Most popular songs have Guitar Pro tabs with complete tracks, i.e. all instruments tabbed. You can use this by removing the guitar tracks, exporting the tab file to MIDI, and then applying your nicest VST intruments to those MIDI tracks.
1) Find a good Guitar Pro file (.gp3, .gp4, .gpx) of the song you want. Ultimate Guitar and Google are your friends here.
2) Open the file in Guitar Pro, and export it to MIDI (it’s in the File menu). If you don’t have Guitar Pro you can use the free application Tux Guitar instead (haven’t tried this myself, but theoretically results should be the same).
3) Start a new project in Reaper.
4) Select Insert -> Media File and select your exported MIDI from step 2.
5) Delete the guitar tracks. These are the ones you want to record yourself of course.
6) For each track you keep (usually drums and bass), apply a VST instrument using the FX-button of each track.
The result is going to sound a lot better than a MIDI file played off of your computers soundcard MIDI bank. Another advantage of this method is that you can make changes to the tracks, for instance pan instruments, change their volumes, add humanization for more groove, and even edit notes if you think they are wrong. These tracks will also be included in your general mixing, so if you add an overall reverb to your mix they will be included and not doubled (like an imported backtrack with reverb might be).
For illustration, I made a super quick backtrack from a GP5 file with Drumkit From Hell and Cakewalk Studio Bass.
Soundcard MIDI version:
DFH and Studio Bass version:
In laptop speakers the difference isn’t that big, but when played in headphones or on monitors you’ll really notice the difference. Now I only need to get DFH sounding exactly like I want :-).