anything is possible in DR. my neighbours have dobermans, one black, one brown. unfortunatley they obtained them by the means of adopting dogs directly from the street after rightful owners kicked them out (leaving the country) so i do not know where did the doggies originally came from. but pet stores in santiago or SD should have puppies available from time to time.
be prepared to pay the price thou, much more common and popular rotties can reach more that 10k pesos (depending on pedegree).