The least expensive single action airbrush you can find. You can find them at craft stores or the hobby section of a department store for around 10.00.
If you go with a airbrush get atleast a dual action as you can have more control but they are a little bit tougher to use it first. I use my touch up spray gun for mine and it works well just make sure you have a bunch of them to do at once or its kind of a waist in my opinion.