On ebay from time to time. And NW transmission parts for synchro's.
Transmissions 1940 to 1950 have many common parts. The synchro~drum may be gone.
On pg 65-A items a, b, c & e wear out.
Also check the engine & torque tube rubber mounts. (rough clutch & into 3rd)
Link --> https://forums.aaca.org/topic/254772-39-buick-clutch-removal/ Use a clutch pack for a 1995 Jeep Grand Cherokee 6 Cyl.