What is a substitute for cornstarch ?
Potato or rice starch, arrowroot, quick-cooking tapioca, instant mashed potato flakes...all can be used as thickeners.
depends what you are using it for- coating for frying? replacing flour? thickening a sauce or soup?
Since I have a sensitivity to corn (can't have too much) I never have cornstarch in the house. I always sub potato starch.
If you are thickening a sauce, even all purpose flour will work. Sprinkle a few teaspoons over the simmering liquid and whisk it in. Let it cook for another minute or two. Actually, any starchy flour will work, and I match the flavor of the flour to the main dish, e.g., cornmeal to thicken chili, rice flour for stir fry.
For thickening pie fillings, all purpose flour will work as well.
However, in baking, cornstarch gives the final product a crispy, crunchy texture that can be hard to duplicate. Potato flour will give some crunch but a more delicate texture. Rice flour may be closer to it; I haven't used it enough to judge. I'm thinking arrowroot might be the closest sub; will have to try.
I use "Instant Clear Gel" of King Arthur Flour for baking (e.g., thickening pie fillings etc). It does a really good job without the sticky/chewy feel of corn starch, which I do not like. Not sure if you are asking the question because of an allergy. It just says "Modified food starch" on the label, so it might still have some corn in it (their customer hotline is great, if you'd like to ask them). It is by far the best product I have tried for baking.
For cooking purposes, I sometimes sub potato starch for corn. You need to use less potato starch than the corn starch the recipe calls for. I'm not sure about the exact ratio but I would start by putting half the amount and adjusting from there.