I too tried to buy just the battery, but I couldn't find anything. In the end I asked for a replacement because it was faulty, and they changed it for me under warranty.

You can buy the light as a spare part:
https://www.mrmaria.com/de...
should be this one

If you have the necessary knowledge, I would open the lamp and order one with the same/similar battery via Aliexpress and solder it in.

The spare part as a whole will cost you about 30Fr with shipping, a small battery from China is about 5Fr with shipping.

You can charge the battery via USB.