If you live in America, stores like Target, Wal-Mart, Walgreens, and Toys 'R' Us might still carry Tama Gos, if at all. In my experience, toy stores have slowly been phasing out Tamagotchis, putting them on clearance and such. Anything older than TMGOs, your best bet would be Ebay, unfortunately.
Look at garage sales. I got a blue V1 at one for.... 25 cents. So they do not really know the price. Good luck finding them. Also, I remember seeing and buying a giant wave 2 lot at Party Packagers.
Also, look in flea markets and antique shops. Very frequently, people bring them in with loads of other "junk" items and you can get them for reeeally cheap.