There's tons of reputable breeders out there but your decision should come down to your goal. Your primary purpose for the dog should effect your decision. Not all GSP's love the water or sitting around in a duck blinds, not all GSP's are built for Chukar hunting. You'll want to meet the Sir & Dam, possibly hunt with them or at least reach out to people that own dogs from past paired breeding's to ensure the pup will meet your demands.
If your looking for an all around pheasant/waterfowl dog, the last thing your going to want is a high strung GSP built and bred for Chukar hunting. Size and weight will also be a factor, along with others obvious things such as health.
From what I've learned you get about 50% regression in breeding's. Take two dogs that range 1000 yards and you get a 500 yard dog. If you find a breeder you like, focus on repeated breeding's so you have a good idea of what you'll get.
High Desert Shorthairs is the breeder I used for my last GSP. His dogs are breed and built for Chukar hunting and they can go day after day.
I'm pretty new to the GSP world and by no means an expert, these are just some of the things I've learned so far and will use when searching for my next GSP.
