I would imagine that a lot of the cardboard ones probably got trashed/recycled. They're not the kind of thing that people would think are really worth selling or donating, especially after nobody gives a damn about them anymore.

Somehow I largely missed out on the phenomenon. I never owned any, don't recall ever seeing anyone playing, and I was only vaguely aware of their existence (like from the Nintendo Power supplies catalog selling them and what have you). Maybe they weren't nearly as popular in my neck of the woods.