Yep exactly, that's my experience as well. And I think the other reply saying it's possible to find them on eBay is kind of missing the point. Anywhere you can buy them is kind of obscure or unofficial.

I don't understand why something so cool, that keeps working forever, is so relatively rare.

I think the only other one I've seen in the wild since that exhibition in the 90s that wasn't some tiny thing, was a Lord Of The Rings one that Weta Workshop had given to a lecturer of mine.