Kevin Deal at Upscale Audio is a good source for tubes. Some aren't any or much more than stock.
When I bought my first ARC preamp 13 years ago (the LS-1), I knew nothing about tubes. It's a hybrid linestage that uses one (1) 6DJ8/6922/7308 tube. (In fact, I still have it) My dealer helped me obtain two replacement tubes from ARC for $50 each. That's a lot of money back then. They were just plain low noise Sovtek 6922s. Yeah, after that, I decided not to get suckered again. I've since learned as much as I could about tubes and I've not paid that kind of price for any tube, including NOS Siemens CCa, Amperex 7308, Mullard 10M 6DJ8, since then.
There are lots of good tube dealers online. You can find some on the homepage of Audio Asylum listed as sponsors. If you are not into NOS tubes, you can find good quality, tested, matched, low noise current production tubes like Sovtek and Electro-Harmonics from these dealers for like $10-$20 a pop. But NOS tube is the way to go. You will never go back once you try them.
And there's a bonus with playing NOS tubes. You can always sell them for more than what you paid for. I think they are better investments than most of my stocks. Within the past 5 years, I think the prices of NOS tubes have at least doubled or tripled.
I was thinking more along the lines of your Sovtek and Electro Harmonix advice, and wondering how tubes like that, from a third party supplier, would compare in price and performance to the ARC stock stuff.
Re NOS I guess I shouldnt knock it until I have tried it, but somehow my brain puts NOS tubes in the same category as cryoed power cords, green markers on the perimeter of CDs, mpingo pucks, and anything ever sold by Michael Green.
The more reputable dealers will go through the same rigorous process of testing for noise and matching tubes. So no issue from buying from tube dealers. "Third-party" is probably not the right word to describe these dealers. They get the tubes from the same source as all the audio manufacturers. And you pay less, actually, then what the manufacturer will charge you, in most cases.
Regarding NOS tubes, it's more like "they don't make them like they used to" The golden age of tube was back in the 1950s and 1960s before transistor was introduced to the world. Now adays, it's just a small nich market, and all the art and know-hows have been lost. And the tooling too.
We just replaced a pair Sovtek 6922s in my friend's Sonic Frontier Line 3 with a pair of NOS tubes (Amperex 7308 USN-CEP white label) last month. And all four judges had their jaws dropped to the ground. One guy hasn't stopped talking about trying to locate a pair since.