Can’t buy output tubes anywhere except from manufacturers at high prices.

These are available and are excellent sounding EL34's:

https://sophiaelectric.com/products/pages-se-el34

Sounds like maybe I should plan to sell a bunch of KT88 Gold Lions I haven't used in year.

I just sold two pairs of GL KT88 matched quads on USAM - both quads sold within one day of listing.  I no longer have any amps that run them.  I apparently didn't ask for enough!  So yes, clearly the demand is real and the supply is low.