Yes, i tried to find them, too. The Alpha ones in the digitakt can be found at shady chinese websites, but they seem rather pricy (can be as high as 15€ per piece!).
I guess they are great for human machine interfaces, where absolute precision is not important - but high resolution is. I have heard about those multiple times in the recent months, but never seen one for sale anywhere. I guess it's still a niché type of thing - though I would think that this could change in the next years, because they seem to appear in more devices recently.