The battery of the Twine Garage sensor lasts for around a year or a few hundred open/close events. When it’s getting close to zero capacity, a warning will be shown in the Twine app notifying you to change the battery soon.
The battery is a standard 23A battery (12V) that you can find in most supermarkets or electronics stores. If you look for “23A 12V” on Amazon you will find plenty of options as well.