If you're in Europe, GPD can send one to you, but it'll cost you 60 dollars with the shipping costs. Additional batteries cost 25 dollars so if you find people for a group order, you can lower the shipping cost.
You can also try looking for alternate batteries on Aliexpress. They'll be cheaper, including shipping, but they're likely to be of lower quality. Some people tried the "Liter" batteries, but they seem to be having lower capacity (less uptime) and, more problematically, voltage issues (meaning your WIN might switch off when load is too heavy for the battery, especially when the battery isn't fully charged).
