Some replacements for potato starch are cornstarch, arrowroot and tapioca. These are all thickeners that can give the same result when substituted in a recipe that requires potato starch. Potato starch is a gluten-free thickener used in soups, stews and gravy.
Although these different starches can be substitutes for potato starch, there are different advantages and disadvantages with each of these substitutes. For example, if one is substituting potato starch in a dish that is acidic, then one could choose arrowroot instead of cornstarch. The reason for this is that cornstarch loses some its effectiveness with the addition of an acid.
Similarly, a substitute for gravy that has milk as an ingredient can be cornstarch instead of arrowroot. It can be a better choice because adding arrowroot to a dish containing dairy makes it slimy. Tapioca is a useful substitute for potato starch if one needs to thicken a sauce quickly.