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

I just bought Audio Research REF 750s mono amps. I believe it has 21 tubes in ea amp and it has 20 KT150 in ea amp. 40 - KT 150s total. The original set has 600 hrs on them and I also bought a brand new matched set of 40 KT 150s from the seller that he purchased from Audio Research as a spare set. So I will definitely not be worried for some many many years to come. 

All over Amazon, my guitar supply. Depends on the tubes.

At my guitar spply I got quad JJ KT77s Matched Apex from there at a decent price.

Viva Tubes has them. The prices are up but,  the price of everything is up now.. 

Viva Tubes has them. The prices are up but,  the price of everything is up now.. 

They've been out of KT120 and Gold Lion KT88 for a while now. And their supplies may be dwindling on remaining Russian KT88, EH KT90, and KT150. Mullard KT88 are still available for now (up to matched octets), and are excellent. Better get those while you can, if you're in need. I bought a couple octets for my VAC amps.

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.