What and where can I get the battery?

"tl;dr

You do not “need” a lithium battery. Alkaline or silver oxide are both cheaper, with silver oxide lasting longer than alkaline but probably not as long as lithium.

They each have different voltage discharge characteristics, but luckily the A-series cameras have voltage regulation (which is what makes alkaline OK to use here) so that the shutter works as long as possible and that the meter stays accurate until the camera shuts down.

explanation:

All Canon A-series models (and New F-1) use the same battery type, specified by Canon as either Alkaline or Silver-Oxide.

1) 4SR44 or PX28-S, modern silver-oxide replacement

2a) 4LR44, “28”, modern alkaline replacement

or

2b) four LR44’s (stacked together) are the same thing as one 4LR44 or “28""

3) PX28L, “28L”, modern lithium replacement

Just be aware that searching for one results in hits for the others, so pay attention to the actual chemistry if you care about that.

Depending on the model, the battery is checked by either observing a blinking LED, listening for rapid beeps, or looking at a needle in the viewfinder. Slow-flashing LED or slow beeps mean your battery is about done. On the AT-1, the needle will just keep registering ever lower during the battery check function."