In general, it's better to buy from a reputeable breeder, or from a REPUTEABLE pet store. The word reputeable is the most important one. There are bad breeders, just as there are bad pet stores. There are many more bad pet stores than breeders, though.

Fauna's BOI is one place to go to check out breeders. Local breeders and herp societies can probably tell you if any of the local pet stores have the OK. In the Omaha area, I can vouch for Too Many Feathers pet store. The owners have some rough-looking animals in, but these are generally animals that people have given to them, which they are vetting and nursing back to health. They make sure all of their ball python hatchlings feed before they put them up for sale.

The local PetCo also has decent, clean cages and healthy-looking herps. Apparently their reptile person knows what he/she is doing. The ball pythons are probably captive-hatched, but they are fat, well-hydrated, and have shed well. I would not hesitate personally to buy from either store, if they had something I wanted so badly I was willing to pay that price for it.

Pet stores run the gamut from absolutely awful hell-pits to decent, clean establishments with healthy animals. Obviously if you get a snake from a pet store, and it dies, and then you get another, and it dies, either their snakes are sick, or you're doing something wrong.

The most common mistake is improper heating. Thermostat-controlled heat is not optional.