Where can i buy battery for my i phone 6s

You need to make an appointment at the nearest Apple Store for Battery Replacement. If you replace with a non-Apple battery, some functions on the phone may not work.

Apple Store - Find a Store - Apple

iPhone Battery Replacement - Official Apple Support

Nowhere. Apple does not sell batteries to the public.

You can take it to an Apple Store or Apple Authorized service provider and they can replace the battery in the iPhone for a small fee.

In the U.S. its $49 for a 6s battery. Link-> iPhone Battery Replacement - Official Apple Support

You cannot "buy a battery". Apple does not offer iPhone replacement parts.

You may ask Apple to service or replace your existing battery.

iPhone Battery Replacement - Official Apple Support