I make my own icing sugar all of the time. I don't see why more people wouldn't, and then I thought...maybe they don't know how. Here's the recipe for icing sugar!
1 cup of white sugar
1/2 tblsp flour
1) Blend all of the ingredients in a blender until it turns into icing sugar. Make sure that you really grind it to make sure you make a good consistency.
* 1 cup of white sugar = 1 cup of icing sugar, however, I find when you make your own it is slightly sweeter, so you might want to add slightly less to any recipe.