The latest Tamagotchis can usually be found at your local Toystore, so get one while they last (^__^).
Pixelmood Shop
Sometimes I put Tamagotchis from my personal collection on eBay, I hope to help people with their collection. Click here to go to the auctions »
Online Stores
Tamagotchi Shops StoreThe older Tamagotchis and Virtual Pets can be found at Ebay. You can also search at local websites where people sell their stuff, like Marktplaats in the Netherlands or Craigslist in the USA. You'd best look at the seller feedback to see if a seller is reliable, I usually don't deal with sellers who have less than 98%.

Nice sellers:
- Jason is an Australian who lives in Japan, he sells great Tamagotchis and cute japanese VPets, like Mutsu (in a round tank)
- Tamenagerie: Melissa sells very nice virtual pets she has bought too much of.
Other Options
- Keep an eye open when you're on vacation, for instance I saw some virtual pets in Turkey.
- Also visit flea markets, people will sell their old toys for very reasonable prices (^__^).
- Tamatalk has got a trade and sell section.