I don't think DJI has ever carried them (they'd rather you pay for something new). But if you google "dji mavic 2 controller battery" you should find links to sellers on ebay and other places, and you'll also get hits for "how to replace" videos...
I got it to hover in the living room and then set it back down. I guess it's good to go!
The cat was not happy.
I've played with the settings and now I'm sticking it back on the charger for the night. I'm going to try a short flight tomorrow...