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

Viva Tubes and Brent Jesse are the ONLY vendors I bother with.  Excellent service, sales, shipping.  Brent goes out of his way to help even with the utmost, novice, actually stupid questions.   I know....I asked a lot when I acquired a near mint MC-225 built in 1963 a few months ago.

I have a pair of Sophia Royal Princess 300B, special edition, listed on USAudiomart, if that's what you are looking for.

Don at Wathen audiophile may be able to help, he carries Cryotone tubes.

I haven’t noticed any issues in fact, I’ve noticed prices going down or at least stabilizing as inventory levels regain ground. 

JJ makes the best EL34 and KT88 tubes IMHO.

https://www.eurotubes.com/store/pc/home.asp