Where to buy original MacBook Pro 13" battery?

Nowhere. Apple does not sell MacBook Pro batteries to the public in any capacity.

You can take the Mac to an Apple Store or Apple Authorized service provider and they can replace it for you.

Any batteries you fine in places like eBay or other online stores, are either counterfeit or scavenged from other used Macs.

13-in MacBook Pro 2016 and 2017 models are still serviceable by Apple. That service includes a new battery, installed and warranted for the standard 90 days of every Apple service procedure. Cost are typically under US$250.

In some cases, the parts replaced may include the keyboard, trackpad, and top plate as well, because that is the standard field-replaceable unit.

You're welcome.