A breeder who does all the necessary tests related to their breed prior to breeding. Do you homework. AKC, UKC, CKC, etc. AKC will register a white, dysplastic, aggressive Boxer as well as a nicely bred Boxer. And this is only as good as the breeders.
Some unscrupulous breeders use false papers to register puppies.
White Boxers CAN be registered. If it is going to be a pet only, then no papers will be given. White Boxers should NOT be bred. You should not intentionally breed FOR white puppies. While white Boxers are just as healthy as their colored littermates, they have a tendency to be more sensitive to the sunlight and do not meet the breed Standard due to their color.