The old mercury type of PX625 / PX13 battery (used in some types of Minox flash, such as Minox C4) has become obsolete a long time ago, they are not produced anymore. However, you may use an adapter to get your Minox flash going, and it apparently consists of a brass adapter, where a 1.4V Zinc Air battery can be put inside and it becomes a PX625. Neat. Batteries to use are: Zinc Air 675A, 675, A675.
If you still wish to use dedicated PX625 batteries, you can buy a Varta V625, which are still foundable. They're 1,5V, but actually they go straight to down to 1.45V once you put them inside your flash, at least this is my experience.