According to Joyofbaking.com, when cornstarch is used for thickening, it is acceptable to replace it with all-purpose flour using a ratio of 1:2. Therefore, if the recipe calls for 1 tablespoon of cornstarch, 2 tablespoons of flour can be substituted.
Other dry ingredients are also possible replacements for cornstarch. A 1:1 ratio is used for potato or rice starch, rice flour or arrowroot. This means 1 tablespoon of any of the powders can be swapped for 1 tablespoon of cornstarch. Quick-cooking or instant tapioca also work in place of cornstarch at the same ratio as all-purpose flour: two parts tapioca equal one part cornstarch.