Your are right not to buy from Pet Stores, but puppy mills also sell to puppy brokers who have web sites, the Humane Society recommends that no one buy their puppy from a web site. You need to see for yourself the condition in which the puppy's mother and father live, that's the only way we can really help stop the suffering of so many dogs. Here's a link which explains the differences between a byb and a reputable breeder, to me, the main difference is that the reputable breeder is actually trying to improve the breed, while the backyard breeder is trying to earn a few extra bucks.
Lawrence County Humane Society Abuse and Adoption Center