Make your own powdered sugar
Last week as I was making cupcakes for the Cub Scouts, I realized I didn't have quite enough powdered sugar for the icing. It's definitely not worth it to me to pack two kids into car seats and run to the grocery store all for one cup of powdered sugar, so I improvised.
I put 2/3 cup of granulated sugar in the blender with a heaping tablespoon of cornstarch and blended it on high for a few minutes. While the result wasn't exactly like powdered sugar, it was close enough to do the trick. And I think that if I had a better blender and if I had let it go longer, it would be even better. It's worth a try, especially if it saves you a trip to the store.