Besides its most common use in making royal icing for cookie decorating, meringue powder adds stability and improves the texture of the buttercream. If you struggle to make good buttercream, I'd encourage you to read my comprehensive post all about American Buttercream. You can buy meringue powder on Amazon, or shop for it at your local craft stores such as Michaels, Joanne and Hobby Lobby, as well as on the cake decorating aisle at Walmart. Meringue powder has a long, stable shelf life, so it will last you quite a while.