Due to time constraints, I just bought a sprinkles pack from a big box store, but many of our local Amish/Mennonite run stores carry decorating and baking supplies for very low prices. (My container of green decorating sugar cost 68 cents).
I make a thinned out buttercream icing for the cookies and paint it on. Since it is so thin, you don't need much. It dries hard overnight. As you can see in the picture, it is runny and will run off onto your table (and dry hard overnight) so be sure to clean up before bed.
To make the icing, I used 2tbsp softened butter, 1 tsp vanilla extract, about a cup of confectionary sugar, and enough milk to make it thin enough to paint. Add this 1 tbsp at a time, but I'd estimate I used 3-4tbsp. Just mix in your mixer and add the milk slowly. If you add too much, counter with more powdered sugar.
Just paint the icing on with a spatula or brush, add sprinkles, and dry overnight.