Where can I get a piece to amplify my exhaust size from 2.25 ID to 2.5 ID and up?

Usually any type of quality exhaust shop can supply you with that adapter... or try a metal fabrication company.

Or... you can just weld on a Header Reducer backwards... Don't see why that wouldn't work?

Jeg's (http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=102159&prmenbr=361) has an assortment of collector reducers.

Thanks, would a 2 chamber flowmaster sound any good on my car, I like it loud, I know those are loud but would it sound nice?

on a supra?
better off with an exhaust designed for a supra.