Cornstarch. May be called Maizena in Latin stores.
But if used in a sauce or salty dish, you may be able to replace 1 part potato starch with 2 part flour.
You can replace potato starch for wheat flour and corn starch in many recipes that call for just a bit, depending on the application you can likely make the exchange in the other direction. Potato starch is cheap and versatile, invest in a bag of it. A pound can be had for less than $5.
I usually seem to find potato starch in the kosher aisle of supermarkets. Letting you know in case you look for it in the baking aisle and it is not there.
It all depends upon what the recipe is, and what it's for. Why do you want to substitute it?
A fried chicken recipe I'm making calls for 2 tablespoons I have potato starch and I don't have any I do have cornstarch
Agree with June. I do quite a bit of cooking with alternate flours, and potato starch can sometimes figure prominently as part of a gluten-free flour mix/replacement. Generally, corn starch or tapioca starch will substitute just fine, but it is always helpful to have a full understanding of what you are trying to do just in case there is a "twist" that people might not always think of. Not sure where you live, but here in the greater Seattle area, most grocery stores (even the really main line ones) have a "specialty flour" section and both potato starch and tapioca starch are readily found there (corn starch is already on the shelves and easily found in the standard baking aisle). PS - Matt, any flour that is $5/pound, I would consider pricey -- so "cheap" may be in the eye of the beholder (or more correctly, pocketbook?!) ;)
More than half of my family's Passover recipes call for Potato starch. Passover is less than a week away, so why not check the Passover displays at larger supermarkets & stock up.