I am not sure if it was flour or starch, but I saw some powdery tapioca product on the second floor of that large new Chinatown grocery next to Chungking Garden.
Any one of those large Asian supermarkets in Chinatown or on Washington Ave should have it. I think there is a difference between flour and starch but learn from me - go with what your recipe calls for. I bought tapioca starch when my recipe called for tapioca flour and I got a gluey mess.
I'm not positive, but you might try Essene, the natural foods store at 4th and Monroe in Queen Village. They have a fair number of flours and gluten free products.