From experience Don A. Goldberg http://www.dagcamera.com/
and by reputation Sherry Krauter http://www.sherrykrauter.com/
Both have been around a long time. Sherry was Leitz tech, and Don may be factory trained as well. I think Sherry does Leica rangefinders only. Don does the SL and R series and other brands as well. Both have excellent reputations and reasonable fees.
edit: I see Richard beat me by a minute.