The PX625 is a small button-shaped 1.35volt mercury battery which a great number of camera models were designed to use. Unfortunately, they are either very difficult to find or very pricey.
Here are 2 solutions.
1. Use 1.5v LR625 battery instead
The LR625 is cheap and almost in the same size as the LR625. The only problem is that the voltage of LR625 is 1.5v, which is 0.15v higher than that of the PX625. The difference in voltage may slightly affect the light meter. But in reality, it does not matter much.
2.Use Battery Adapter
This copper-made adapter can convert a LR44 battery or 675 zinc-air battery (also known as hearing aid battery) into a PX625. I recommed using a 675 zinc-air battery because its voltage is 1.4v, which is very close to the voltage of PX625, while the volage of LR44 is 1.5v. The light meter is more accurate when your battery has a more appropriate voltage. That is one advantage of using an adapter+hearing aid battery over using a LR625.
Plase note that this adapter cannot change the voltage of the battery. (though some very expensive ones do: they have a chip that can adjust the voltage. I don't think it is worth it)