The most common colour of Dobermann is Black and tan, then brown and tan, then blue and tan, then fawn and tan, then white.
This is because black is dominant to brown and the rest, brown is dominant to fawn and so on. In fact fawn, or Isabella, is a double recessive so very rare. Although it is sometimes nice to have an unusual colour dog, sometimes these rare colours are not so good for the health of the dog. White dogs have particular problems sometimes with eyesight and hearing, and will also be more at risk of skin cancers from sunburn.
Try your local Kennel Association/club for a list of local breeders.