Did you check eBay? There are definitely examples for sale there. Holography-specific groups have hints on where to buy some too. The popular ones are hard to find, but you can make them yourself at home or find less sophisticated pieces for sale occasionally.

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.