Where can i get a battery.

You make arrangements with Apple or an Authorized Apple Service Provider to install a new one:

Genius Bar Reservation and Apple Support Options - Apple

Only at the Apple Store or an authorized Apple Service shop. Expect to pay about $50-70 depending on your model of iPhone.

Find Locations - Applehttps://locate.apple.com › ...

Otherwise, you are going to get a "knock off" battery that will not allow Battery Health and other important settings to work correctly.