I buy dog food at a small store called Especially 4 Pets. It carries high end dog food.
I don't really buy anything from pet stores, but if I do have to go to one I will go to either Petsmart or Petcetera. I will not buy anything from Petland, although sometimes I do go in there to try and educate people.
I have never found people more knowledgeable at one stor over the other. I think that in general they are all pretty dumb, but they do get points from me if they know I have shelties when I walk in. I also think that their grooming and training are horrible (this is only the stores I've been in personally, I do realize that all stores are different.)