If you find any Tamagotchi-related stuff in Japan, you could make a small fortune selling it to all of us.
It would pay for part of your trip!
But - although I hate to be a downer - don't get your hopes up. I have family in Japan who are always on the lookout for Tamagotchi stuff for me, and the craze there is so intense usually that it's simply nowhere to be found. People get Tamagotchi by lining up in stores on the day of release. It's not like here at all, where you can mozy on down to your Toys R' Us and pick it up on sale. Japan is much more trend-driven, so when things are hot, you can't find it anywhere, and when things aren't cool anymore, stores just stop stocking it because they're trying to stock the newest craze.
But good luck, and I really really hope I'm wrong!