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