Iphone 8 plus Battery

If you require a battery replacement for your iPhone, you can organise a service at an Apple Store or an Apple Authorised Service Provider.

Please see the following support article for further information: iPhone Battery Replacement - Official Apple Support

Not as of yet.

Apple recently announced the Self Service Repair program, which will allow users access to Apple-genuine parts, tools, and repair guides. The program will be available this year in the United States, and will eventually extend to other regions. You can find more information about the program here: Apple announces Self Service Repair - Apple (AU)

Apple has "Authorized Service Providers" in many countries that can perform a battery service.

As far as I know, Apple doesn't sell bare batteries. Everything is part of a complete service where they will require that an old battery be exchanged (which becomes their property) as a part of the service. Not sure what the DIY service will entail, but it could possibly include a requirement to send back a used battery.

As far as I can tell, you're in South Africa, and there are "Apple Authorised Service Providers" in SA.