Where can I buy replacement anti-fog inserts for x3 invisible dive case ??

With the size of the anti-fog inserts sheet of the invisible dive case being much larger (even larger than the first-generation x3 dive case), I wonder where I can buy some to keep as a spare. I see that all the sizes that are for sale are small sizes for GoPro, and I still haven't been able to find a size that fits the invisible dive case.