I'll just post the typical secondhand Tamagotchi buying advice here:
Facebook Marketplace
eBay
other local marketplaces (Kijiji if Canadian, Craigslist, Poshmark, Vinted, others local to you that I’m not aware of)
Etsy
Mercari
JapanYouWant
Amazon Japan (you need to create a new account separate from your local amazon account, but you can use the same login information)
Zenmarket, FromJapan, or Buyee: Proxy sites allow you to search Japanese Auctions or marketplaces and purchase items from Japan. They act as a "proxy" for your purchase, completing payment for you and then shipping to you. There are often duties, taxes and fees on top of your initial purchase, but it often still ends up much cheaper than other options for a Japanese tamagotchi. Proxy sites offer first time buyer's guides, which you should read before use. For an example, see here: https://www.fromjapan.co.jp/sp/introduction/en/
You could also join a Tamagotchi Buy/Sell group on Facebook or the Tamagotchi Collector's discord where collectors sell their stuff and where people can put out specific Tamagotchis they're looking to buy.
