Personally I would not let the fact that a club may not have Dobermann's deter you too much. I have trained at a club and have had the only Dobermann for the past 5 years. Good training is good training. It is hard enough to find that without having the requirement of having other Dobermann's there.
I know of two in dallas area dew working dog club and seagoville working dog club. A member of my rawfeeding group in dallas who has rottweilers recommends seagoville the trainer is Angel Ayala but I've never been to either one. Im planning on checking out seagoville though. He has worked with Dobes before.