I love using Tapioca Starch as a replacement for corn starch. This brand is great and you can use it as a 1 to 1 replacement for corn starch.
I buy this product at my local health food store (i.e., Whole Foods). You can also buy a 6 pack of them here at Amazon if you can't find it at a store near you! Love it and if you can't eat corn, use this instead! It will also work great to thicken your Thanksgiving gravy!