Buying OE parts at one's dealer is always a good idea. It builds relationships and gets you high quality parts... not made in China quality. But... this trans place is also very good, it seems. I've never used them, but I like how they only use ZF LifeGuard fluids.
The California Transmission Supply Company
http://www.thectsc.com/catalog/6-speed-bmw-61.html
And here is another place that also sells AT service kits...
http://www.zftranspart.com/