I have found and trained service dogs for people for many years. Here's my experience.
Do NOT get a puppy to train for service dog work. Puppies are a total crapshoot and I have never seen this work out. I made this mistake three times before I got smart. I'm sure it's worked for someone but the odds are against it.
You want a female that is at least a year old or a male that is at least 2 years old. By then, any issues with dog aggression will most likely have surfaced.
The so-called "working lines" are now the worst bet for service dog work. (Wasn't that way 30 years ago.) By and large, "working line" means the dog can do sport, not real world work. Insanely chase a flirt pole, you bet. Prance like an idiot looking at the handler without looking at where he is going, you bet. Lie quietly under a table during an office meeting for three hours, forget it. Wag his tail when a little kid bolts out of nowhere and pokes him in the eye, forget it.
The best dog I ever had was a German working lines GSD. But that was 30 years ago. Now you couldn't pay me to take most working line GSDs. (The reasons why this has happened to the breed is a whole different thread, which I will start someday when I have time.)
It takes a superdog to be a service dog. Forcing a dog to do this work when it is against his nature makes the dog miserable and can be dangerous to the public by making the dog unstable.
Thirty years ago, great GSDs were so common that I could find a good GSD service dog from a rescue group. No longer. I won't even look there anymore.
Lately I've had the best luck with the higher quality American showlines, from breeders who aim for kindhearted, stable, sweet, calm GSDs.
If you can afford it, you could get a GSD service dog from Mace's Malinois:
Breeder, Trainer & Seller of World Class Pedigree Belgian Malinois, German Shepherds, Dutch Shepherds & Miniature Australian Shepherds. Puppies, Trained Protection Dogs and Service Dogs
Most "service dog" breeders are scammers, but this guy does seem to to know what he is doing.