Can i purchase a battery for a 8 plus iPhone

The Apple Store or authorized Apple Service will not sell the battery without also installing it.

$89 U.S.

Avoid buying a "knock off" battery, as it will not display Battery Health correctly and you will be pestered with notices about an "unknown part" in the phone.

Check here.

iPhone Battery Repair & Replacement - Apple Support

Not from Apple. You might be able to get a battery replacement service.