Why would you want to buy a brand new radiator support (core support)? Go to your local junk yard and see there's on there, cut it out and you're good to go. Or, see if anyone is parting their GC8 out. Is yours bent or ...?
IMO used would be the best way to go. As far as I know, they don't make aftermarket new core/rad supports, car is too old. If they do, you'd have better luck Googling it (and see if there is a site advertising it) and go from there, otherwise the dealership, who'll probably want at least 350+ upwards... If not, twice that. My dealer wants $300 for a rear bumper rebar / reinforcement bar..