So far, only the Hood
is available in Carbon Fiber, However, There's a guy with a GTS in my area, I'm pretty sure he's got a Carbon Fiber rear hatch. If I ever get the chance to talk with him, I'll ask where he got it from.
Rear wing is available in Carbon Fiber too, now that I think of it.
Look into getting a titanium exhaust, that'll drop some weight too.
Other options would be to make your own carbon or fiberglass bits, you can buy, fiberglass mat, as well as Carbon Fiber matt (but the carbon fiber isn't that cheap). Make molds of your fenders, door panels, whatever, and use the molds to make your own pieces, it can get pretty involved though, so it's not for the timid. I plan on trying my hand at making a set of Carbon Fiber front fenders this summer, that is, If I can find a cheap supplier of Carbon Fiber mat. the guy's I've found so far charge way too much. I may just opt for fiberglass instead. Regardless, I'm pretty sure someone wil make aftermarket fenders and such eventualy.
Oh yea, if you have an ABS or FRP bodykit, replacing it with a fiberglass kit will shave some weight, as will ditching the spare tire and jack, ditching the back seats, or replacing your front seats with lightweight racing seats. You can also replace the bolts on the passenger side seat with old-school Hood
pins, that way, when you race, you yank the hairpins, pull the seat, and you're ready to go.
Lots of other things you can do too, just get creative.