Hi Dogman; first off, you should contact our member Coastie99 via PM using
this link to his member's page. He has a stash of 6GW8s and 6BM8s; I know he'd be glad to help you out. The T1 RVT has two 12AX7s in the preamp, two 6GW8s for primary output, a 12AX7 and a 6BM8 in the reverb amp, and a 6CA4 rectifier.
6GW8s: they have been out of production for some time and are only available as either NOS or used. The 6BM8 has been brought back by Electro-Harmonix and is available new if Coastie can't make you happy. These two tubes types were developed in Europe for the post-war table radio market; both types are two tubes in one bottle ... a triode section like 1/2 of a 12AX7 and a pentode section used for output. US manufacturers fooled around with both but none of the domestic stuff can touch Siemens / Telefunken / Mullard / Amperex / Tungsram for tone. I don't know what Coastie has in his stash but these makes are the top of the food chain for both types.
12AX7s: there are as many fans (and detractors) of new ones as there are brands of new ones. The NOS/used Euros - see list above - command the big prices ... $75 and up ... followed by '50s/'60s RCA black plates for $45-$60. If you have the money, you should consider putting some of it into one really good 12AX7 for V1 - the first preamp tube. If you have any money left, the next chunk should go into the reverb 12AX7, and whatever's left into V2 which in that amp, is more of a worker bee. Finally if you like bottom-fishing, there are hundreds of good-sounding, low-mileage GE, Sylvania, and RCA gray-plates on eBay for $10-$15. You can get 5 of them for what 3 new 12AX7s cost increasing your odds of getting several inexpensive, good-sounding keepers in the group.
The rectifier tube is inexpensive and widely available:
6CA4
As far as new tubes go, I had good results tubing up a Gibson GA18 preamp with a pair of premium gold pin JJs; musical, articulate, but a little bright to my ears ... a condition I fixed by putting a new JJ 12AT7 in V1 ... I thought they were worth the money. Good luck with you amp.