Yes, there is a website that sells backing tracks, and of pretty good quality too. They can be found at
http://karaoke-version.comThey have a vast library of all sorts able to be downloaded in mp3 format. All their songs can be previewed in their online mixer. After you pay, the song is displayed with individual tracks, which you can mix online before you download. You then download your mix as a stereo mp3 track.
However, you are able to mute individual tracks in the mix, leaving just the track you desire, and then download it. If you do this to all the tracks individually you then have all tracks downloaded and can then import them to your DAW for final mixing.
The process is rather time consuming and the files are largish, because in effect, you are downloading the entire song multiple times with all tracks muted, bar the one you want. You then assemble them in your DAW.
The songs in there library are pretty much true to the originals and are played on real instruments, complete with lead and backing vocals and certainly good enough to use in live performance. Just insert yourself.
Hope this helps.