Howard,
If you are an MBCA member you get The Star. In The Star are three advertisments for transmissions for older MB cars. The first is European Exchange in Hackensack, NJ. 201-489-7770.
The second is John Olsen in Minneapolis, 612-377-0156; he advertises manual trans only, but my guess is he's got contacts for automatics as well. Worth a call.
Then there is Silver Star 800-369-6601. I don't know where they are located; they advertise "electronic transmissions" so might not do older ones, but can probably point you in the right direction.
Bud's 800-942-8444 has auto trans listed on exchange, with an "inquire" note about price and availability.
Usually the advertisements are worth the cost of joining alone. So, you have 3 sources.
Of course there is always the Classic Center.