you can get them at
-walmart
-shopko
-target
-walgreens (though actually, i've never seen one there)
and several other places--just go to the more general stores AND the toy stores... sometime you'll find one. but as they're fairly new, VERY popular, Bandai is having trouble keeping up with the demand, and they aren't released in some places, it might be awhile. plus, a new wave of colors is supposed to come out in a month or two, so no place will be restocking. bandai might not even make teh old colors anymore.
(i got the info about the new colors coming out from several places--tamagotchi.com has the colors there, and some website--i forget which--has them on pre-order, they ship late march)
ebay can be perfectly safe if you go about it right, but only if you're megacareful, and it can get a bit expensive.