[ghaff]

Some older cameras that did have replaceable light meter batteries also used mercury cells which are likely hard or impossible to find. There may be size compatible non-mercury substitutes but they probably won't have the right voltage characteristics to work well.

[armadsen]

There are both adapters for Silver-oxide batteries, as well as Zinc-air batteries specifically designed to replace Mercury cells for cameras. See https://shop.criscam.com/products/mr-9-mercury-battery-adapt... and http://www.weincell.com. Some light meters can also be recalibrated or modified to measure properly when using a Silver-oxide or alkaline cell.

[ghaff]

I knew that silver-oxide batteries would often work. I had forgotten about the zinc-air hack although they, of course, expire fairly quickly once you start using them.