The forum's site sponsors will be a good start:
http://www.bmw-sg.com/forums/categories/bmw-specialist-workshops-and-servicing.165/
Of course there's always PML and MA
Previously for my 5ver E60, it was the mechatronics solenoids problem. Replaced those together with fresh ATF and all good to go like new. Not too sure about the transmission for E65. It may not be transmission/gearbox issues for your case, depends.
http://www.bmw-sg.com/forums/threads/mechatronics-solenoid-replacement-for-ol-e60.76359/
Good luck
Thanks for the reply.Look for Kanash from Riverview Auto AMK Autopoint.
It may be just the solenoids