They are in Sharp Denshi Notes. They don't sell'em outside Japan and they don't support English. They also aren't really readers but more like digital agendas or notepads. I had one for a while but my model couldn't export a notebook as a single PDF, instead a bunch of pngs, which I found annoying. 6" is also a bit cramped for writing. It also wasn't super visible except in the brightest of rooms and direct sunlight.
The screens can also be had for devices like the Raspberry Pi. I don't think they make good readers though. The high refresh rate is not needed and they really need insane amounts of light to be viewable. There's a reason eInk won that space.
https://www.adafruit.com/product/4694
eInk on a watch is not pratical for a watchface with a seconds counter or second hand.