Potato starch is an important component of gluten-free flour mixes. For those living in Shanghai, Carrefour Supermarket sells potato starch for about 4-5 RMB per 330g bag. The starch is located near the salt/seasoning section. You can find MSG, sugar, and other types of flours in the same aisle.
The Chinese translation for potato starch is 马铃薯淀粉. In case your computer / browser does not read Chinese encoding, here’s a gif image of the Chinese characters.