iPhones battery

Apple doesn't sell or distribute any original internal component to anyone, except to their Stores or Authorized Service Providers. So, you'd better contact them.

At your own risk, you can go to a 3rd party repairer, ask for replacement, or buy a new (generally not original) battery, keeping in mind that, if something goes wrong during this replacement, Apple won't assist you, 'cause that iphone has been tampered.

Regards

Giulio