A question about a recipe: Chicken Meatballs with Tomato-Balsamic Glaze

If the above answers aren't satisfactory, and you an a food processor, you can make it yourself. Remove all the skin off the chicken or buy it sans skin, also boneless maybe to helpful. Prepare the chicken by cutting into one inch pieces and placing them on a small tray. Cover with plastic wrap and place in the freezer. Leave in the freezer for 10-15 minutes, that pieces should be hard on the outside but if you press them hard they yield slightly. With the blade attachment, place half the chicken pieces in the food processor and pulse until the chicken is in small uniform pieces (1 second, 8-10 times), remove and repeat with the second batch. This is create more like chopped chicken, but it will be completely usable for the recipe.

If you have a meat grinder, either hand turned or as a Kitchen Aid attachement, you can grind your own. If you don't have one, perhaps you could borrow from a neighbor.

Otherwise, Miranda's suggestion is great.

You should be able to buy ground chicken at almost any butcher or grocer - if they don't have it, you should be able to ask the butcher to grind some for you!