Southern Gear, in West Atlanta(?) has rebuilt Getrags in the past. They are NOT inexpensive. (I received an estimate of over $1k. This was over 5 years ago.)
There was another outfit in Florida who sells rebuilt units, but I forget their name.
Another option is a ~'93 "Beretta Getrag" aka New Venture Gear T-550. It is essentially a later generation Getrag. Bolts in to the stock Fiero Getrag mounting brackets. It uses a hydraulic release bearing, instead of the usual slave cylinder and release fork. The differential gears are sturdier than the Fiero (and other earlier) units.
I have one bolted to my 4.9, and really like it.
I can post a link, later on, if you are interested. I can't get to that forum from work.