Dingoonity.org

I don't ship internationally but for those of you in the US
https://www.ebay.com/itm/GPD-Pocket-Replacement-Battery/312812855732

For those of you in other countries GPD's secondary aliexpress store has it:
https://www.aliexpress.com/item/4000570018059.html

And for those in the EU I'd imagine distributors over there will be getting them at some point or another and they'd be cheaper than the Aliexpress listing, they aren't listed as in-stock right now but it might be worth shooting them an email to see.