I would rank your options like this:

1. Buy from a reputable breeder
I've never had a problem with a snake I bought from a breeder. Not all breeders are great of course, but if you ask questions you can get to know them and figure out how they keep their animals. A breeder can tell you more about their snakes than anyone else. It's also safer and less stressful for the snakes to go from one home to another, vs. being transported from the breeder, to a show or a store, and then to the new owner.

2. Reptile show
This is still a good option and you'll have plenty of breeders and snakes to choose from, but be wary of unwanted hitchhikers. Always use hand sanitizer before handling and make sure you shower once you get home. You don't want to bring home mites or anything. The other downside about shows is sometimes they're so busy that you may not have as much time as you'd like to ask questions.

3. Local pet store
Depends on the store. Some are better than others. If you can smell the place before you even enter the doors, turn around and walk away. But if not you'll want to go in and get a good look around, make sure the cages are clean and the animals look healthy. If everything looks fine handle the snake for a few minutes and ask questions before buying.