Sweet rice flour
Flour ground from glutinous rice, a particularly starchy rice variety; once cooked it turns chewy, elastic and sticky.
Sweet rice flour is ground from glutinous rice, whose starch is almost entirely amylopectin; despite the name it contains no gluten. Kneaded with water and then boiled, steamed or fried, it forms a chewy, stretchy, slightly sticky dough of the kind found in rice cakes and mochi. The taste is mild and faintly sweet, and the texture is what matters. Ordinary rice flour looks identical but behaves quite differently, so it is worth checking the label carefully when buying. It is sold in Asian grocery stores, often by mail order, and increasingly in larger supermarkets.
Substitute:
There is no good substitute. Ordinary rice flour or wheat flour will not bind into a chewy, elastic dough, and the result turns crumbly or short; tapioca starch adds some chewiness but is more glassy and tastes different.