In Japanese cooking, we often use potato starch (we call it Katakuriko 片栗粉) to thicken sauce, coat the food before deep frying/frying, or prevent from sticking (in case of mochi). For Karaage recipe, I highly recommend to use potato starch instead of corn starch for the wonderful texture potato starch gives. You can buy online here (but much cheaper in Asian or Japanese grocery stores).
Substitute: Use corn starch.
