The windows are no longer available from GM. You might find a new set on ebay, but you'll have to pay through the nose for them. There are aftermarket windows which are good, if you're not a stickler for exact detail...they all have little differences in them from the originals, but you also won't pay the price of the originals, either. You might also keep track of this thread here on Pennock's...
https://www.fiero.nl/forum/Forum4/HTML/030265.htmlMsLoriFiero is working on making an exact replica of the GT window. She's run into some problems, but they're working on getting them ironed out.