Prepare a 13"x9" pan with cooking spray; set aside. Preheat oven to 325. In a large skillet, cook chicken broth, onions, and celery until vegetables are tender. Add black pepper. Bring to a boil. Reduce heat. Cover and simmer six minutes. Add 14 ounces cornbread stuffing. Mix well. Place mixture into prepared pan. Sprinkle chicken evenly over top; set aside. In a small bowl, combine sour cream and chicken soup. Mix well. Spread evenly over top chicken.Sprinkle top with remaining stuffing. Bake, uncovered, for 30 minutes, or until heated through.