It's awesome that you did your research and joined this forum BEFORE purchasing the snake, most people don't even do so.
The best places to buy a Ball python would be from a reputable breeder or a reptile expo/show.
You can check online for breeders that live by you or you can search for breeders around the country.
At reptile expos/shows you can walk around and check out everyone's booth to see what they are selling. Personally I like going to expos a lot more because you get to actually see the snake, handle it, and check it out before you think of buying it. Plus it's so much fun to talk to vendors and have them share their knowledge to you, and they will answer any questions you may have.
If you try purchasing from a breeder or going to an expo/show you can see what you really want. Plus the quality will be a lot better since they are produced from breeders. I've noticed that breeders tend to sell higher quality animals (even if it is a normal) and you will feel so much better knowing that you are purchasing a healthy snake.