I finally found it: A blue bathroom stool, which is apparently called a ハユール (hayuuru) after the company that is most famous for making them, Hayur. I searched in several shops. One had them, but not in the color I wanted. Which is blue. After having seen these little plastic chairs both in animes and at every onsen or sauna I’ve been to, I have concluded that they are as central to Japanese society as the tea ceremony.
As I wrote in September, both 1980’s anime Ranma½ and the more recent Ichigo 100% contain a blue hayuuru in plastic:
And now my bathroom does as well!