There was a UK company originally making medical tubes that came out with updated things like 12 series with considerable improvements a number of years back. Never heard of them since.
Basically, the market is tiny and the cost is high if I were to venture a guess. One can emulate the distortion products in a transistor circuit and using various FETs, do a pretty good job of the transductance curves. I don't think we understand the "musicality" well enough to emulate in DSP but I bet some are working on it.