The problem is less of finding the tubes, and more of finding properly tested and balanced tubes.
Raw bulk untested 6N3 tubes are super easy to find for reasonable cost if you have a tester, not being one of the ultra popular tubes and all. Russian NOS of the 6N3P-EV are fairly cheap on eBay (~$45 for 50 count as of last month when I bought em), and the 6N3 is current production in China and are readily available.
Better yet? You can use a bloody GE 5670W, the most amazingist of the enhanced 6922 designs (with a different pinout). These are super easy to find balanced and tested for under $25 (tube depot), or just in bulk at $4-5 a pop (eBay).
That said, the output power is complete anemic (crap), and if this was any brand other than SMSL, I’d wonder if they’d just thrown the tube in for looks.
I personally wouldn’t buy one... but that is totally not because of the tube choice.
(edited for readability)