Please do not support a place that sells puppy mill dogs. They will tell you that their puppies do not come from mills, but I have friends who breed high quality, imported german shepherds and great danes and both of them told me that no reputable breeder will sell their puppy to a pet store. EVER.
Breeder that cares about the wellbeing of their puppy will spend considerable amount of money and time on proper health testing including OFA/Hip, Penn-Hip, CERF-Eye, cardiac screening, Von Willebrands Disease and others. They also make owners sign a contract for the dog to be spayed/neutered at certain age and if anything happens, the puppy goes back to the breeder. Absolutely no rehoming.
If the "breeder" sells to a petstore, they obviously could care less about where their pup goes which is a huge red flag.
The store won't give out the names of these breeders, because they are commercial breeding facilities where parents are kept in dirty, cramped cages and are forced to pump out litters several times a year.
Please do your research and spend little extra money by supporting a reputable breeder or rescue a dog from a shelter (there are plenty of puppies in rescues needing homes).