Ebay is my best friend for sure, especially for the vintages. I also mostly get my Tamagotchis through here or Tama Zone. Occasionally, I use Hobby Link Japan.
Usually, New... From Target, Walmart, TJ Maxx at random (I think) then Ebay I guess... Mostly Target though. Sometimes I get them from ToysRUs, when I am over there which isn't that often....
I've bought from Ebay all my Japanese Tamagotchis. Most of my english Tamagotchi's were bought at a NZ shop called The Warehouse, and an Aussie and NZ shop called Toyworld. I also have two from a New Zealand second hand online shop, called Trademe. Though the items I buy from there are second hand, they were in good condition.
I mostly buy mine at the store, they were selling them at wallgreens and I have bought them at wal-mart and target but I have just recently, today actually, bought them on amazon!
I've gotten mine at various places, such as K-mart, Walmart, Walgreens, Target, etc. Even Goodwill. Ebay for Japanese and vintage. And Toys R Us when tamas were first introduced in 1997.
I get most of them on ebay ( japanese ones) and I also get them from safeway becuse that is the only place they stock the tama-go and sometimes I find old ones (v3,4,5 at oldest) at toyworld and toykingdom I also find some at op shops that's probaly the best place for second hand ones