In half power mode, V6 & V9 aren't doing anything anyway so it won't matter what tubes are in those sockets or if there are even tubes in those sockets at all. If you don't plan to use it in full power mode, you don't even need to bother with ordering tube(s), unless there's some other problem with the tubes. Here's a schematic if you care to see how it works. When you switch the amp to half power, the cathodes of V6 & V9 are removed from ground so they do not conduct.
P.S. Welcome to the place!