eBay is a good starting point.
https://www.ebay.com/itm/NEW-G3HTA001H-Original-Battery-For-Microsoft-Surface-Book-Keyboard-7-57V-60-8Wh/282806460954?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D5518ff3e14e84484a53854f5b4f758f8%26pid%3D100675%26rk%3D1%26rkt%3D15%26sd%3D282806460954%26itm%3D282806460954%26pmt%3D1%26noa%3D1%26pg%3D2380057&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A8a0f89ab-0aaf-11ea-986d-74dbd1806121%7Cparentrq%3A8301ce3c16e0a4b7b6da448affeb87be%7Ciid%3A1