Toys R Us and Wal-Mart are the best places to buy tamas, but also check Target, Zellers, and [superstore (if you live in Canada)] they sell them for lower prices.
Hmm, well the most popular stores where I live are Toys R Us and Target. I have also seen many of them at K-Mart but I have seen over 30 in Toys R Us so that is my #1 recommendation. But you can also get many different and rare types of tamagotchi on E-Bay and Amazon has a lot of tamagotchis too! lol Hope this helps!