A reputable obedience trainer should be able to help you with this, it's one of the basics. Off leash reliability is great, but you should always keep the law in mind and don't break it even if your dog is obedient.
Do you have a GSD? Perhaps contact your local GSD club(s) and ask for recommendations. Plenty of trainers will have good reviews when they train easy pets, but a trainer that is reputable with working dog clubs will likely know how to handle strong willed dogs, which is important with of leash training.