Substitute for 6F07 tubes.

I am new to tubes. I bought a CJ Premier 11A amp. It uses 6550, 6F07 (6FO7?), and 5751 tubes. What is the modern substitute for the 6F07 tube? I don't need replacements yet but wish to prepare for the future. I don't see the 6F07 tube available anywhere or mentioned anywhere.
It's a 6FQ7, as indicated on this page, or, equivalently, the 6CG7 as also indicated there.

They are easily findable in vintage form, and I believe some recently manufactured versions are also available.

-- Al