I always like to have piping bags on hand, so I highly recommend buying a big box of them (it's not expensive). You'll find that you can use them for all sorts of things besides decorating cookies.  But if you need a substitute in a pinch, a sturdy gallon-size zip top bag will do the trick.  Another great decorating method that doesn't require a piping bag is to simply dip the cookies in the melted frosting! It's simple, but it's pretty.