Apple does not sell their genuine spare parts to non authorized Apple Service Providers. If you find a spare part battery on the internet, it is mosts likely used, if you can find any credentials from Apple on it.
...
Beware of counterfeit parts
Some counterfeit and third party power adapters and batteries may not be designed properly and could result in safety issues. To ensure you receive a genuine Apple battery during a battery replacement, we recommend visiting an Apple Store or Apple Authorized Service Provider. If you need a replacement adapter to charge your Apple device, we recommend getting an Apple power adapter.
Also non-genuine replacement displays may have compromised visual quality and may fail to work correctly. Apple-certified screen repairs are performed by trusted experts who use genuine Apple parts.
...
copied from: Official Apple Support