What batteries do Canon IIIa cameras need? Or do they need any? I've never used a film camera before. I've tried googling but all that comes up is about DSLRs.
You mean this one from 1952?
Batteries in those days would have been nearly the size of the camera. You can be pretty sure nothing of that vintage is ever going to need batteries, except for a flash,
which wouldn't really be on rangefinder cameras for another few years.