Not all pet stores are bad. I bought my hamster from Petsupermarket. He was about 6 weeks old when I got him. He lived to almost his 2nd birthday, an average lifespan of a golden hammie. What most people do with their hamsters (Not saying you did) is they either overfed/free feed those junk food that comes with seeds and crap, provide no exercise wheel or any form of exercise, stress and dirty cage (contributing factors to "wet tail" which can kill them without vet care) or nothing to chew on so that they can keep their scissors short or else it will grow to the extent it can't eat and starve.
Secondly, you may get lucky like I did and bring home a wonderful snake from a terrible pet store. Just make sure you check its health completely, including rubbing against the snake to feel for any bumps, check for mites on your hands as well as on the snake's eyes and its body and any sound it is making while breathing or mucous from anywhere. If you bring home mites by accident, it is annoying but fixable. Good thing is that this will be your first snake so you only have one to worry about.
Lastly, though they are few and far in between, there are good pet stores out there. It is good to start checking out different pet stores and find one you can trust, even if you don't buy your snake from there, in case you need advice, help with equipment or pick up a feeder. A good pet store will get to know you as well and some may even special order certain things for you (ie enclosures) whereas most online retailers won't do that.
Ps the 20 gallon is too long. My adult male bp is in a 56 qt tub and I want to give him more space in the future. It is awfully crowded after you out in the hides and water bowl.