I recently bought a Canon Ex EE (1967) but there isn't a battery. I read from the manual that it used PX625(1.35V) but there isn't any available now. I do not want to buy zinc-air cells. There are PX 625A(1.5V) and LR44 also 1.5V available. If I use these are they harmful to my camera, such as burning the circuit board? If not then which one is better?
There's technical information here about the various substitutes available that give you more options. The page lists several substitutes with information about availability as well as technical information regarding voltage stability.