There are a number of reputable breeders that are recomended in the links area. Lots of people here have orded thier BP's from a mail order breeder and have had nothing but great reviews. Another place to get a ball python is a shop that deals only in reptiles then you could hand pick the one you like. There are some things to consider when buying a ball python.
First and formost if you see them in a tank with no water bowl and no hides and there is a bunch of poop and old sheds in the tank go somewhere else for you ball python.
When you do find a place look for the following:
When you handle it, it should want to explore and flick its tounge actively and should grip your hand when it moves it could "ball" when its first handed to you but when it feels safe it will start to investigate you.
Check it closely for ticks or mites and other parasites.(especially around the eyes)
Check the skin see if its soft and smooth if its wrinkly and rough its sure to be dehydrated.
Check their belly scales and make sure they are not broken or orange in color they should be white with a touch of grey or black pattern.
Check their mouths to be sure there is no mouth rot this looks almost like cottage cheese on their lips.
Check their eyes they should be black or blue-grey if its going to shed. If you see any dents in the eyes the animal is dehydrated.
You should not hear any wheezing sounds when its breathing. This means a respiratory infection.
In a perfect situation you want a captive bred and born individual and these can be rather pricey but are well worth the money. On the other hand you could get an imported Ball Python but these tend to have problems not all of them but maybe a hand full and they are cheaper i bought mine for $20 each. Both of my BP's are imports and i have not had any trouble with them. They both eat like champs and have never refused a meal. They are docile and have never tried to strike at me. I think the key to getting an import is to get them when they are really young.
But when you go look at some ask the person helping you some questions.
When it last ate and ask if they have records of the feeding if they dont ask them to feed it for you right there and then if they refuse walk right out of there.
Ball Pythons are easy to care for if you provide the appropriate requirments you should have no trouble.
www.bobclark.com is one of the best places to buy a ball python its mail order but they will send you pictures of the ones they have to offer. And all of his ball pythons are sent to you nice and well fed.
Remember even though you buy the best ball python out there always take them to a vet along with a fecal sample just to make sure you got a healty buddy.