Allen Wright (Vacuum State Electronics) is really the only one I know of that will be selling them, though I'm not sure they're avialable quite yet. He's doing differential circuits with solid-state current sinks.

The big leader in this field has been Lynn Olson, you can find info on his amplifiers here: http://www.nutshellhifi.com/triode1.html He doesn't sell anything though. His webpage is amazing, look at everything. I don't think there's anyone that knows this stuff better. He generally promotes 'normal' push-pull over differential because the output tubes are in parallel instead of series then.

Gary Pimm has been working on some push-pull stuff as well. (Just google his name) He did a differential all-pentode amp that achieves something like 10hz - 100 khz flat, limited by the output transformer. OH and it also sounds pretty good too :)

I think K&K audio has been fiddling with push-pull class A triodes too but nothing available yet.

It's funny to say this is 'cutting edge' since it's really like 60-70 years old, but the revival of this stuff is very recent (2-3 years). Previous to that SET dominated all tube sales. I imagine in the next 3 years or so we'll see more commercial products. The first ones will be from Allen I expect though.

Things were looking bleak for tubes, but between this excitement, and the fact that some REALLY good tubes are coming out of China these days (TianJin/FullMusic 300B's, 2A3's, and even 45's current production are awesome)... well, things are looking up.