Does anyone still sell a complete rebuild kit for w124 transmissions?
 

One that includes gears and bearings?

1991 300d 256k

Automatic transmission kits usually include seals, clutches, gaskets, clutch steels and possibly bushings and bands. Bearings, drums, pumps and gears a/k/a "hard parts" are usually replaced only as needed often from used donor transmissions due to their co$$$t.

Where to buy? Try Fatsco in Pine Brook NJ and it would be worth asking Sun Valley.