Separate wireless Fender guitar controllers are due out next month for the 360 and PS3 for $60 each. Not sure what's going on with the PS2 stuff.
Or another route would be to buy the bundle, keep the game and guitar, and eBay the mic and drums. Doesn't look like any of the pieces are available separately for the PS2, so I bet the mic and the drums (especially) go for a decent amount, probably to people with the game only Rock Band and one of the GH SG guitars.
I'm having a lot of fun with Rock Band, but I've kind of flown through the guitar stuff so far. I'm looking forward to getting the drums at some point, which will be a hell of a challenge and I'm sure will make it seem like an entirely new game to me.