If you remember, my husband has a corn allergy. The only powdered sugar that I could find was made of sugar and cornstarch. Thus, for a little over a year, I've been grinding sugar in a coffee grinder and mixing it with potato starch. It's a tedious process. But I did it because I truly believed that ALL powdered sugar had cornstarch in it.
Whole Foods, bless them, has made a powdered sugar out of cane sugar and tapioca starch! It is all organic and it is GRRRRRRRRREAT!