I got mine from summitracing.com, believe it's Walker 40094 is the part number. Free shipping. Beware there are two catbacks available and the difference is the number of exhaust outlets (the GT's all used two tailpipes) and if you find it much cheaper than roughly $140, it's probably the single-exit system.
I went back and looked at the PartsGeek link and it says they are out of stock. Summit is a little pricier, but apparently still available. Fiero Store is also out of stock.
I am sorry to revive this thread but I found that the WALKER 40094 exhaust kit is no longer manufactured by Walker and they are not making a replacement for it either.
I searched for an exhaust system on summitracing/partsgeek/autozone and none are available.
Does anyone have another website or resource where I might be able to purchase a kit or am I better off just getting a new crossover pipe and rebuilding the exhaust 1 section at a time?