I used to buy my guitar controllers with their respective game (on amazon and such), it was always cheaper than the controller alone. I don't know what to do with the spare GHTV games, though...
It probably depends on where you live, but it seems every Goodwill in eastern Massachusetts always has at least 1. I was just at a Goodwill yesterday that had 2!
Apparently I am missing out on a Guitar Hero renaissance!