What's the best substitute for glycerin in making royal icing?

My baby's 1st birthday is coming up and I plan to make cupcakes with royal icing as frosting. However, I couldn't find glycerin to prevent the royal icing from drying up quickly. I need to frost the cupcakes in the morning, because I still have lots of chores to do, but the party will be in the afternoon. What's the best substitute for glycerin?