Well I just replaced mine with the Dorman aftermarket cover. I'm pretty sure you could find an NOS one around since this was used on ALOT of vehicles. The Dorman cover $20.00 Advanced auto is not as thick as the GM one but mine was very pitted and wanted to clean it up back there. FYI bolts are 20lbs torque cross pattern I used the Felpro gasket instead of the one that came with the cover. It takes 2Liters of fluid and bottle of GM additive.
that same cover was used on millions upon millions of GM cars and trucks from the mid 60's to well past the '00 model year that used the 8.2", 8.5", and 8.6" differentials- it was even used on the front differential of 1/2 and 3/4 ton 4X4 trucks from 79-90 or so.