Okay, did some poking around: apparently Avago/Broadcom was the only major manufacturer of single-chip optical mouse sensors, and they shut down that entire product line a few years ago. So that's why there aren't any DIY mice now.
@phooky I'd be interested in a trackball.
@grantcuster which reminds me that the reform 2 has a trackball and the designs are open! It looks like it's using a pixart chip, but once again we're back to calling distributors, etc. I feel like if I wanted to build a 1-off I'd be better off just disassembling an existing mouse.
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