
I think your best bet would be to contact some of the GSD breed clubs, and get a longcoat pup from a standard coated litter bred for work or show, then you are more likely to find a pup who has been bred with reference to the breed standard, rather than the mating of two pet dogs that bear a passing resemblance to GSDs.
Also you would expoect generations of health testing in such litters. Hip Scoring and Heamophilia testing of the sires is esential. Of course yo7 need to satisfy yurself as to temperament, and bear in mind that there are at least two types of show GSD, and some very nice middle of the road ones too.