Edited January 18, 2021
at 11:50 AM
Accidentally found this while looking for a battery on samsung.com
Samsung Wireless Charger Portable Battery [microsoft.com]
(probably 15W, even though not specified, even on the order confirmation page)
The cheapest price which I could find.
Shipping is free.
PS. Please be gentle - first-time poster, but a long time user.