Is there an alternative to cornstarch when used to thicken a pie filling
There are several alternatives, but the most common I know of are tapioca flour and arrowroot powder/flour.

That said, most cornstarch substitutes aren't appropriate for pies, because they break down under high heat.  The only substitutes for corn starch that I'd use in a pie are potato starch and tapioca.  Tapioca powder can be hard to find in some areas though; I usually get it at Asian grocery stores.