Dealers most likely are supplied through one or more rebuild "factories". I know BMW almost exclusively uses ZF USA to rebuild transmissions. However, I was told they use other suppliers.
Personally, there is only one person I'd send my MB, BMW or Euro trans to for repair, parts or rebuild. IMO, between price, service, quality and honesty there is no better guy. I'll supply the contact info via PM or Email if anyone is interested.
I and many other RoadFly listers had fantastic results with repairs/rebuilds. My experience was with a 740IL trans rebuilt. If anyone is interested I'll post my fairly detailed experience.
BTW, I have no relationship with the abovementioned business. I'm merely a very satisfied customer.
'85 380 SL (sold)
'85 Carrera Flatnose
'71 280 SL Signal Red/Cognac