Missing a bolt somehow. What’s a good place to get one?

 

5/16-18 x 3/4", nothing special, a grade 5 will do, not a high torque requirement. Anywhere that sells bolts, hardware, big box store like Home depot, Lowe's, Menards.

 

5/16-18 x 7/8 GM part # 563592 if you want an original. Salvage yard, GM dealer, driveline shop, etc.

 

IIRC a Powerglide, TH350 or T400 pan bolt is also the same. Vortec 4.3/5.7L water pump pulley bolt would also work. If you are doing the scrap yard or garage crawl. I like the OE GM trans pan bolts because they are serrated flange heads in many years and help prevent them from backing off. I upgraded my 14 bolt FF to the trans pan bolts and used RTV specific for gear oils and oil pans to keep the cover sealed up nicely. Never will put a gasket on a rear end.