If you don’t have cornstarch, you can thicken the pie using tapioca flour or all purpose flour.  For tapioca flour, add the same amount as the cornstarch and let the mixture sit for at least 15 minutes before refrigerating to allow the tapioca flour time to soften.  If you use all purpose flour, add double the amount of all purpose flour.  Note that the all purpose flour will change the appearance of the filling.  It will be more cloudy and if you use too much flour it can affect the taste of the filling.