As far as makers of Jankó keyboards that you can buy, i found two, there must be more though.
There is a piano maker who does Janko keyboards: Reinert in Germany They make upright pianos. There is also a video with some good playing.
ADDED: I love the ‘vertical slides’ at just past 1 minute in this video. there is a vertical chordal slide to the right and one to the left, different but similar. I think this would be unplayable (certainly at this speed) on a standard keyboard.
Then there is Daskin Manufacturing in Reno Nevada, who makes a Jankó midi controller (or he did fairly recently). The company is run by Paul Vandervoort.
As the video shows this thing is really nice. (I really like the four rotary knobs, i’ve never seen anything like it before, almost like a rotary slider. Very nice!) Lots of controls on this thing besides the keyboard. No idea on the price.
You can also see a guy, Noel Cragg, perform on this controller. The player is very good but the sound quality of this video isn’t.