Tapioca starch is widely available, so you can purchase it local grocery stores, health food stores, and online. One of my favorite brands is Authentic Foods brand tapioca flour (affiliate link). I usually buy Authentic Foods tapioca starch from Amazon since I'm a Prime member and the price is usually quite good, but you can also buy from their company website or from Vitacost.com. Be sure to search around for the best price, which varies at any given time.