Um, ok... that's kind of redundant. Where do you think Petco and Petsmart get their mice? From breeder/feeder shops... Breeders supply the local pet stores. I own a pet store, but I sell to several pet shops around me, and a couple of states over. That's pretty much how most pet shops get their feeders. I do know that some Petco's and Petsmarts buy from wholesalers around the country. But, again, most wholesalers buy from breeders...
So, in short, most mice come from small commercial breeders, such as myself...