Japanyouwant is your best bet. Prices there are often way cheaper than ebay, they are one trusted seller so there's no way to get scammed (though sometimes they do sell used items and there were some incidents where some faults weren't described but that's a rare occurence), the shipping ist reasonably priced and fast as well as reliable. They are a dedicated store with tamagotchis of all kinds, while on stores like hobbylinkjapan or cdjapan you'll only find the newest releases (which aren't restocked after they run out of their stock, usually.)
As long as you know the gist of ordering something online, you'll be fine on JYW.
Though if you are lucky and manage to be early enough to get something before the preorders close (that's for new tamas only) then hobbylinkjapan, cdjapan and amiami are the cheapest since they usually sell for under the MSRP.