[Freya]

There will possibly never be a affordable way to change a tube in a vintage set now, the NOS tubes on ebay are likley to make decent money because of that.
As more enthusiats get into vintage television restoration any remaining new vintage stock will dwindle rapidly and we will be faced with making do with what we have.

You are faced with getting the set running to prove the tube good, no point in sourcing replacements or a spare at this stage IMHO