None are currently being made, and probably never will be made again. Your options are: E-bay, recycler, turbo buick boards for sale/trade options, or Brian weaver at GBodyParts. Or some real GNX rims at $$$ HTH
I have always been told that if you choose to use GTA rims they have to be front only. Not sure if this is fact or not. Like someone already pointed out check www.gbodyparts.com but hang on to that wallet. Last I looked they were $300 a rim.
The reason you need four gta wheels is because the offset is correct with the front ones. If you look closly at the rears you can see that there is a larger polished lip and if you try and use them on the rear they will hit the lip of the wheel well. I looked hard and ended up paying $950.00 for a set of four refurbished gta wheels. I live in the cleveland ohio area and I purchased them from a place called motomotion. I hope this helps