I originally came across these optical ones...think they might be out of production and selling off old stock, though I'm not sure. (I'm also not certain if these are entirely self-contained, or if they're just part of the sensing mechanism.)
https://wooting.store/products/analog-f ... s-big-pack
I contacted them via the address on that page to ask, and they suggested the Hall-effect ones instead, although I don't see them advertised anywhere. But they let me order half a dozen which are apparently now on the way.
Here's a blog from a few years back where they describe the different technologies.
https://blog.wooting.nl/analog-input-te ... keyboards/
They produce "gaming keyboards" where individual keys provide a range of analogue values for use with compatible software, but potential applications seem much wider, and I think they're hoping they'll catch on.
It's very difficult to find light, comfortable buttons which can be used to deliver velocity information, so they could be onto something.
* * * *
Assuming they work...what's the best way to sample the state of a couple of hundred of them with a Teensy to get note-on velocity values?

(The original Axis used a custom silicon-rubber overlay with concentric carbon contacts. Velocity was calculated from the timing between the two consecutive connections.)

*edit*
...oh, looking at those keys again, I don't reckon they include the Hall-effect sensor, just the magnet that activates it. In the same way, I think those Flaretech optical keys only have half the equation.
Never mind; still interesting. If this is the case, you can pick up bundles of Hall-effect sensors fairly cheap if you know what type you want. Hopefully they can advise.