1.3v mercury batteries are hard to find. I bought some on a trip to SE Asia a few years ago - they were expensive, and the clerk in the store refused to give me a sales slip. Apparently, selling them in that country is illegal and he didn't want a paper trail.

The Wein air cell is supposed to be the best replacement, but it has a problem. It uses the same technology used in the batteries used in hearing aids - they are sealed at the factory, and the seal must be removed when they are installed. Once the seal is removed, the life of the battery is brief - a few days at most. That may be what the dealer installed for you.

Depending on the specific battery required, it may be possible to purchase an adaptor that will allow you to use a smaller silver cell - basically a spacer that allows a physically smaller battery to fit into the camera. This will have longer life but the voltage and aging characteristic will be different.

I have an Olympus 35RC that uses the PC625 mercury battery (that I bought in SE Asia). The current battery is going strong, and I have a few spares (one of the more significant advantages of mercury batteries was the almost infinite shelf life), but when I run out I have an even older hand-held selenium-cell light meter that I plan to use with the camera in manual. You might consider that option.