Reputable breeder or breed rescue would be my route. Shelter if you get a pup. Stay away from backyard breeders, breeders who "specialize" in more than one or two breeds (i.e., puppy brokers) and definitely STAY AWAY FROM PET SHOPS. They buy their dogs from brokers who get them from the mills.
Pits are great dogs, but there are too many idiots out there who breed them for the wrong reasons or want them because they think they're some sort of "macho" dog.
I have a friend who shows boxers and also participates in agility and flyball with them. According to her, there are a lot of white boxes in rescue; of course, many of those are deaf. I don't know if an excess of white in a Staffie has the same genetic problem, but I believe an all white head is to be avoided.
Do your research. Find out what health problems these dogs have and make sure (if you go to a breeder) that the proper health tests have been done. If you go to a breeder (you're probably more likely to find a pup from a breeder than a rescue, but older dogs still bond and have many advantages over a pup) make sure the pups are raised in the home and not a kennel. Visit the breeder and meet the pup's dam. The sire probably won't be there, but if so, meet him, too. If their temperaments are off, chances are the pup's will be, too.
A good breeder will ask a prospective owner as many questions as you ask him or her. So will a rescue. Don't be offended.
I have GSDs myself. Distanced, I loooove Newfies.
LOL Leave it to a dog question to bring me out of lurkdom.