Wein cell zinc-air batteries are an exact drop replacement for the original mercury cell PX 625. There are also similar hearing aid batteries. The hearing aid batteries are smaller than the original PX 625, but there are work arounds. GOOGLE knows.
I own a Luna-Pro and 2 M5 bodies. They all take PX 625 batteries. I have used the Wein cells in all three. They work. They last about a year in the Luna-Pro. I found a stash of originals and use those right now. I carry 2 Wein cells in my bag for spares in the field.
$37 buys a lot of zinc-air batteries. It will buy even more hearing aid batteries if you can get them to fit.
Good luck!