Where I get original iPhone 12 Pro Max battery in goa?

The only Apple Authorized Service Provider in Goa servicing iPhones is PCSS Solutions Private Limited. They won't sell an iPhone battery, but will only provide one as part of a complete battery service including an exchange of the existing battery. I can't seem to post the phone number (some issue with automatic recognition to keep from posting private information although this is a commercial business) but you can always look it up.

PCSS SOLUTIONS PRIVATE LIMITED

SHOP NO. 15, GERA IMPERIUM GREEN

NEAR JAIRAM COMPLEX, NEUGI NAGAR

PANJIM 403001

https://locate.apple.com/in/en/service/?pt=3&lat=15.3514906&lon=74.0680165&address=Goa

You will have to find an Apple Authorized Service Center near Goa and have that establishment replace the battery for you.

Axel F.