Ummm.... Ebay? You know there really isn't a shop, but if you type whatever "old" tamagotchi you are looking for into ebay, I'm sure you can find someone who is willing to sell it. It really depends on what you are looking for. Are you looking for P1/P2 tamagotchis, v1, v2, tamagotchis, angelgotchis, etc? Here's a link if you're looking for a v2 tamagotchi (or at least I'm pretty sure it's a v2, because I haven't seen that specific design on the v1):
Click here!Good luck and take care not to buy any costly fakes on ebay! :D