Yes, you can buy those little squeeze tubes in the grocery store, however they contain some of the worst-tasting stuff you’ve ever put into your mouth. Really. Better, I think, to make one’s own. For home bakers, I simply suggest a thicker version of the powdered sugar-and-water icing below. It’s not much different, save for a little less water, about 1 tablespoon and 1 to 1 1/2 teaspoons of water, plus color of course.
To be sure, for the truly serious there are more sophisticated techniques for making decorative icings, but this is a solid way to go. However I will warn you that detail icings that are heavy in color (say, if you make a very dark brown, black or green one) will tend to bleed into the base coat. I therefore suggest that you use lighter colors to for your design work. Do that and your results will be almost indistinguishable from bakery-bought cookies.