I have looked ALL OVER Tokyo and Yokohama for stuff like that, but nada. I've found some of the happy turd toys in Vancouver gomi shops, but I'm not buying Japanese toys in Canada when I live in Japan.
I did buy a nice book called "Everyone Poops", but I think it's available everywhere. On a slightly serious note, a lot of Japanese kids are scared of the washroom, especially at night. We have the bogeyman, Japanese kids are told about some monster that will grab them and pull them into the toilet. Their diet makes a lot of them constipated, too.
edited to correct the book title
