I got a old Canon EOS 500 (analogic) of my girlfriend, but I need to buy batteries, but do not know what the correct battery for her, found a manual on the internet and said CR123A or DL123A, but found no amperage specification (3v, 3.7, 4v ).
I sent an email to the canon but got no answer.
You actually have all the information you need. CR123A is what you need to buy. They happen to be 3V, and up to 1500mAh, lithium chemistry but not rechargeable.
You should be able to find them in a camera shop, a chemist, or even a large supermarket, but it's easy and cheap to order them online.
There is a rechargeable in the same shape, with a slightly higher voltage that might still work, but getting a charger for it isn't easy, so the batteries are rare as well. In an analogue camera they last ages (many, many rolls of film when I used to use a compact that took them).
CR123 is a Lithium battery with a nominal value of 3.7 volts. If you were to measure the voltage it would be between 3 and 4. Being a battery the voltage will change as the battery is used. (It can even change based on temperature, age and many other factors.)
There are tons of different names and numbers for the same battery… DL123a, CR123, EL123, K123 are just some examples.