I know people extoll the benefits of getting a snake directly from a breeder, but even though I breed corns I only sell a very few directly, most of mine wholesale to local shops because I don't want lots of visitors!
I'd shy away from buying online, personally, unless it was from a strongly recommended source. I have ordered corns from Lexcorn and couldn't praise the health of her stock highly enough, but in the UK you have to pay for a reptile courier as animals can't be sent through the post. I have heard horror stories of the couriers transporting multiple animals and disease being transmitted because they don't quarantine properly, so I travelled to a reptile show to meet her and collect my corns. (Of course they can't make a trip for just one animal, they'll transport a vanload and drop off on the way)
SO if you can't make a personal visit to a breeder, I'd say choosing from a well-run reptile store is fine. Ask to see the hatchdate and feeding records of the snake. If you are told something like 'oh we feed every Sunday', walk away. You will have no guarantee that that particular corn is a good feeder. Check the viv or display tank hasn't got dirty water bowls and lots of poop. Check each baby is kept separately in a small tub, not a big pile in one tank. Ask if the snake has been sexed or if they can do it for you in the shop.
(I sell each of mine with the hatchdate, feeding record from hatching and sex from popping. I can guarantee males but females are always unproven IMO)