How Common Are 21FJP22 Tubes?

[compucat]

I have been reading a lot of posts about CRT rebuilding lately and I was wondering about the rarity of the 21FJP22 round tube. I just won Harry Poster's auction for the round tube Zenith. I am pleased to know that Hawkeye does rebuild these (I inquired just yesterday.). Are working tubes or good duds easy to find? I know I will want a spare, maybe two for this Zenith after I get it restored. I don't have the set yet. I'm waiting for Harry to get back to me with a shipping quote.