You shouldn't have to constantly rebuy them, certainly less than yearly restocking and you should have plenty lasting longer, but a lot of the popular ones have larval stages which simply won't survive in a reef tank without moving the new larvae into another vessel and feeding them live foods. Given the inexpensive price of most of these organisms and their plentiful and quickly replenishing place in nature which allows them to be sustainably harvested in many cases, people generally haven't prioritized aquaculture of them with all of its difficulty and expense.
There are some that can reproduce in captivity in normal tank conditions, and if you want to try to raise them, most types will still lay eggs in a home aquarium, so it may be worth trying for your own interest, but covering all the standard CUC member bases with something that will reproduce in a home tank without collecting and raising the larvae may be pretty tough.
As mentioned, trochus, dove snails, and stomatella snails are all good options, but you're going to have a real tough time finding any crab without a delicate larval stage and I don't know if sand sifting snails or conchs can be had that don't have that same larval stage.
