audiotubes.com previously owned by the great Brent Jesse has a great list and selection, and hopefully the new owner carries on his passion and legacy.
Go to https://www.audiotubes.com/12ax7.htm and review Brent’s commentary on various tubes and sonic characteristics.
You’ll pay a premium for Mullard (worth it IMO), but not the most expensive. RCA triple mica 5751’s are fantastic too. Amperex and Brimar are great.
Implementation in the gear matters, so the rules are fluid.
Note that some NOS tubes are not branded or labeled as expected, for example I just sold a pair of Mullard’s, one is branded "Heath", and the other is Mullard, but they are both the exact same tube made in the exact same factory.
Personally, I never bought into the idea that there are different levels of new production tubes ranked like American Express cards.
So the key is a trusted source. If I was still using any tube gear I’d give the new owner a shot. Brent used to have a solid return policy, not sure if that remains but if it does you have nothing to lose.

