I'd try Create / Synergy...
NOS Sylvania are still reasonably priced, I picked up a quad of chrome dome GTAs for $95 recently. It would take more work to buy NOS tubes, and replacing bad tubes might be an issue as well...
Also, maybe the old Reflektor 6H8Cs, they are readily available, I got a quad for $25, haven't compered them to the Sylvanias yet though.