Anker, the brand that makes charging accessories for mobile devices, also has a mouse. "Vertical ergonomic mouse" appears to be available in wire or wireless, and left-handed or right-handed models. I'm using a right-handed wired version right now, and even after a few minutes, my wrist feels much better.

One of the trainees in #AL had one (violating the rule about attaching one's own equipment to $EMPLOYER devices), but I did get to discuss how it felt. He said he'd purchased it at Walmart, so I went to both local stores without finding it. After I returned to #CA, I checked the smaller of the local stores and also checked Target. I ended up ordering it from Amazon.