I need a genuine battery for my iphone 6s where i can get it

You cannot buy a factory new OEM battery for any iPhone. Apple does not sell iPhone parts at retail. If you take it to Apple or an Apple Authorized service provider, they will replace your iPhone’s battery and will use a genuine factory new OEM battery when they do the work.

fahamjohn Said:

"I need a genuine battery for my iphone 6s where i can get it: Need a genuine battery for my iPhone 6s"

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Get it from Apple, or a retailer that uses official Apple brand batteries.

Use the Apple locate site to find authorized sellers and authorized service providers -> https://locate.apple.com/

Change the country as needed by tapping on the country name in the top right.

As I said, neither Apple nor any Apple Authorized service provider will sell your friend a battery. They will install new batteries into iPhones when customers bring the iPhone to them for service. But they do not sell parts separately.