I have a relative that teaches in a preschool classroom where they had a hermit crab as a pet. It had to be replaced every 2 years or so as it'd outgrow its shell and reject the ones put in its aquarium as replacements, and then dehydrate itself and die.
Then it became nigh impossible to find new, bigger shells at local pet stores. Do most hermit crabs not live long enough to need them?
I'm sure there's some hermit crab hobbyist site that sells all sizes of shells measured to anal retentive standards. Local hobbyist independent aquarium stores would have extra shells just from die off.
I know there are some aquatic hermit crabs that are huge, but others always stay small. I would be 100% not surprised if the chain stores sell tiny baby crabs that grow huge, as they do with 75% of their fish.