PX625 1.35v mercuric oxide, long discontinued when the use of mercury was banned.
Alternatives: use a CrisCam or a Frans de Gruitjer battery adapter; these are the best option as they use the empty outer casing of a PX625 with a replaceable 1.55v silver oxide battery inside and both have circuitry to drop the voltage down to 1.35v. Silver oxide cells, just like mercuric oxide cells, maintain a near constant rated voltage from the day you start using them until shortly before they die so the light meter on your camera will work exactly as designed and give the correct exposure settings. The adapters also leave the camera unmodified and original with the same original EV range, unlike when you go for the second option below. Also the adapters maintain a good resale value. The CrisCam MR9 adapter is made and sold in the USA but the price virtually doubles if you ask them to ship overseas as they charge a ridiculous sum for shipping. If you live outside North America then the version by Frans de Gruitjer is cheaper and it's technically slightly superior. Frans speaks perfect English and makes the adapters to order by hand and charges very little for shipping. I last bought the Frans de Gruitjer ones at €16 (Euro) each including shipping. At the time The CrisCam MR9 direct from the USA was over £30 per item and them only UK supplier (Small Battery Company) was charging £29.99 plus postage, so I could buy 2 of the Dutch units for the price of 1 CrisCam.
The second option is to get the camera light meter recalibrated to give the correct exposure using a modern 1.55v silver oxide PX625. But that shifts the light meter's entire EV range so it's not as useful in very light or dark conditions.
The third option is to use a zinc-air cell. WeinCell produce a 1.35v PX625 form factor battery. But zinc air cells have a much shorter lifespan of typically just 6 months and that starts from the date you remove the tabs from the air vent holes on the battery whether you use the battery or not. Some users claim that the battery life can be extended by a few months by blocking half of the air vent holes.
The final option is to use a modern 1.5v lithium alkaline cell. The trouble with those is that their voltage discharge characteristics constantly change; they typically start new at a little over 1.5v and as the battery gets used they constantly drift down in voltage. That means the light meter reading is up about 1.5 stops wrong when the battery is new, briefly spot on as it drifts down to 1.35v, and then goes wrong again as it drifts down below that. But if you're shooting on print film rather than slide/transparency film then the printing stage allows you to correct much of the exposure error. That's called "exposure latitude" and the two stage film and print process of print film can be very forgiving but with slide film it;'s simply right or wrong with no correction. But if you digitally scan and then digitally process films or slides then you can pretty well correct most of the errors.