Here is some links for used MB parts and performance stuff.
From what I heard, only MB will do a proper rebuild up here somewhere in Canada, with all the updates to the transmission.
Usually they have the best auto trannys on par with Toyota.
As a simple trivia, the 80s and awesome 500E was assembled by Porsche in exchange for 4 speed autos for the 928.
Consider the fact that changing a tranny is around 5 hours of labor, so if you buy used be really sure that it's good.