Several alternatives:
1) There is a Wein-brand battery that is the same size and shape as the original PX13/625 mercury cell. I find these to be a bit expensive.
2) You can get one of those battery adapters that allow you to use a silver oxide S76/LR44/365 cell. They're a bit expensive -- about US$25, not including the battery.
3) You can use hearing aid batteries, sometimes with a rubber "O" ring from the plumbing area of a hardware store. I think the #9 ring works. You can usually buy a 10-pack of the hearing aid batteries for about US$10, and each lasts about six months.
I'll have to check the battery chamber at home, but I think that a hearing aid battery should be sufficient. Sometimes, these batteries aren't tall enough, and adding a metal washer to fill the gap almost always works.