Honesty I don't think petco is really too evil. A new petco by my parent's old house just opened up. I went to go check it out, mostly to see if they had any kinds of cool wood, and to look at their snakes.
The same weekend I was in downtown Portland and discovered a specialty reptile shop. I was delighted and went right in. As soon as I stepped in however, I was disgusted. I don't really know for sure what rotting reptile smells like but I think that was the odor of the store. My first instinct was that the smell was of death. Lots of snakes, and although most of them were housed separately none of their cages had been cleaned recently and none had water. There were a few turtles and tortoises there with no water either. The man who owned the store was sitting at a computer playing spider solitaire. As I left I heard him say into the phone, "Well, if you absolutely have to go to a vet then I guess go, but...".
Yea, chain pet stores exist, and if you're uneducated enough to buy a snake from one you might end up with a snake that has mites or a kink or is $50 over priced. I feel much more optimistic about the animals that Petco cares for and sells though than that private business that I saw.
I think word of mouth is the best way to make sure that you and the people you know buy healthy snakes from reputable breeders. Let your friends or anyone you hear talking about getting a snake know that there are other options out there.