The plants that made them are probably buried under shopping malls by now :-(
RCA were a big player, ISTR Sylvania, too. The main European producers were STC (Part of ITT), M-O Valve Company (AKA English Electric Valves, or EEV), AEG-Telefunken. Valvo, in Saarbrucken may have been involved, too, but I'm not sure there.
Somebody at the BBC's engineering department (or what's left of it) might give you a steer. Their 198KHz Droitwich transmitter is still running, and that may still be tubes. It's damn big.
Do a Google for
There may be obscure companies manufacturing for the niche market that's left over. If it came to replacing ten grand's worth of final tube or half a mil for a new transmitter, I know what I'd do :-)