How do you thicken stir fry sauce without cornstarch?
An alternative to cornstarch or cornflour is to use plain flour, arrowroot flour, tapioca flour, or potato starch mixed with water.  These all work in different quantities, so make sure to read the instructions.  Even using one or two teaspoons of powdered soup mix or instant potato mash would work to thicken the sauce if that is all you have.