I want to buy a battery for my iPhone

Apple does not sell batteries to individuals. You need to pay Apple to replace your battery.

See this link to schedule a battery repair at your Apple Store Genius Bar --> iPhone Repair & Service - Apple Support

Any Apple Store will be able to supply a battery.

Apple Store - Find a Store - Apple

If you don't have a convenient store, try this link...

iPhone Battery Repair & Replacement - Apple Support

From Apple or an Apple authorized service center. If your battery health is 80 percent or less, you can have it replaced.

Hello bhupendra161!

To get your battery replaced, just go Here to start…

I hope this is helpful. Good Luck!