If you ever need icing sugar and don't have any on hand you can make it yourself by blending regular sugar in a blender for a minute or two.
Last night my wife decided to use up leftover lemons by making lemon bars, but we were out of icing sugar to dust the bars. Fortunately, natural household weblog Keeper of the Home shared the tip on their page for making unrefined icing sugar (or powdered sugar as it's known in the US). The authors of that weblog use naturally-produced sugars, but rest assured that the technique works equally well with plain white granulated sugar.
If you have a smaller or less-powerful blender you won't want to make more than a cup at a time as the weight of the sugar will pack down the blades and overwork the blender motor. Even if you have a performance blender you will still only want to make a maximum of two cups in a single batch. In our Magic Bullet we made a half cup by blending for 45 seconds or so. Also keep in mind that blending sugar could scratch a plastic blending jar -- I didn't notice any scratching on our Magic Bullet, but it might scratch with repeated use.
If you compare the cost per gram of icing sugar to granulated sugar you might find it worth your time to forget the commercial product and make it yourself as needed.
How to Make Your Own (Unrefined) Powdered Sugar [Keeper of the Home]