Here is a useful website: https://www.radiomuseum.org/tubes/tube_6n4p.html
From that page, you can also look up the 12AY7 and 6072. To my eyes, the latter two should be cross-compatible with the 6N4P. (Be careful if you do your own research, because apparently there is also a Chinese 6N4P, and it is not the same as this Russian 6N4P.) The caveat is the filament voltage. For the 6072/12AY7, they can be run on either 6V or 12V filament supplies. I am not sure from the information on the given website that the same is true of the 6N4P, which may require 6V. Still, I don't see why he says you cannot use the 6072/12AY7. The pin connections to plates, grids, and cathodes are identical to those of the 6N4P, and the electrical parameters are close enough. Ask the seller.
From that page, you can also look up the 12AY7 and 6072. To my eyes, the latter two should be cross-compatible with the 6N4P. (Be careful if you do your own research, because apparently there is also a Chinese 6N4P, and it is not the same as this Russian 6N4P.) The caveat is the filament voltage. For the 6072/12AY7, they can be run on either 6V or 12V filament supplies. I am not sure from the information on the given website that the same is true of the 6N4P, which may require 6V. Still, I don't see why he says you cannot use the 6072/12AY7. The pin connections to plates, grids, and cathodes are identical to those of the 6N4P, and the electrical parameters are close enough. Ask the seller.