Where do I go to replace my battery?

Agreed- batteries are consumables, and will deteriorate as they age. The iPhone is just fine. Your battery should be replaced when the Battery Health drops below 80% anyway.

See here for an overview - iPhone Battery Replacement - Official Apple Support

In the U.S., it costs $69 to have the battery replaced:

iPhone Battery Replacement - Official Apple Support

I know that my iPhone 11 Pro is going to need a new battery probably in three or four months. I will definitely be replacing the battery rather than buying a new phone. My phone will support iOS updates for another couple of years so, unless Apple comes out with a new iPhone that has such revolutionary features that I just can't help myself, I plan on keeping this phone for another couple of years.

You may make a different decision.

The iPhone 11 is perfectly capable. You can get the battery replaced at an Apple Store or authorized service provider.

Don't worry- about 1% per month is the standard rate of deterioration.