The only international store that stocks most accessories and components for GPD is Geekbuying so if it's not available there, your best bet is TaoBao. There is everything on there from modded motherboards with extra ram to batteries etc but they don't ship overseas so you;ll have to go through one of the many TaoBao agents-- you need to start by identifying the correct listing on Taobao and paste the URL on the agent's website and they'll order it for you and subsequently ship it out.
