There is a Hokkaido antenna shop in Ginza, near Yurakucho Station. It's called "Dosanko Plaza" (どさんこプラザ)
(The one on the top of the list)
Another branch of Dosanko Plaza is in Ikebukuro. But I'm not sure they sell the pudding balls regularly. I have heard that Dosanko Plaza changes the lineup often. I have seen one selling at the Dosanko Plaza Ginza branch before but it was few months ago.
Another type of Hokkaido antenna shop is "Hokkaido Foodist" near Tokyo station.
I have never been to this one and not sure if they have the pudding balls.