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

We at eastendhifi I have KR Audio tubes in stock. We carry the KT88’s, the 300B tubes, as well as a 300 BXLS from KR Audio.  Yes, they are certainly not inexpensive, but they will sound better than anything out there, other than maybe Western electric, and they will last longer than the JJ tubes. you can check out what we have available at eastendhifi.com.

Having just entered the fray again with amplifiers that use very fragile tubes I have noted that the best prices are available only at the origin of the tubes. A Winged C 6AS7 here is $45.00 but from Russia is $15.00. Shipping for 40 tubes was 20 dollars! Check out TubeTerra. The same holds true for 6NS7s but the very best NOS tubes are only available from Russia for a whopping $8.00 per tube. I had to do a lot of research to find these tubes. You do not have to be gouged by American vendors. Get your tubes where they get them from. 

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.