Hmmm...
How many "USBc" mice ARE there?
I'm going to guess that most makers are still building and selling mice with USBa connectors, simply because the "market is still there".
What I'd suggest:
Buy a wireless mouse. I like the Logitech line, that come with their own "unifying receiver" (which also has a USBa port).
Then, get one of these:
Cable Matters USB C to USB Adapter 6 Inches (USB to USB C Adapter, USB-C to USB 3.0 Adapter, USB C OTG) in Black
www.amazon.com
Plug the unifying receiver into it, and plug that into one of the USBa ports.
Yes, I realize that it's a dongle.
But it should do the job.
There are small USBc-to-USBa converters, but be aware that if you use one, it will likely block access to neighboring ports because of its size. The little dongle above won't do that.