This would be Kool-Aid, a small package (less than a half-ounce) that comes in many flavors and you would want the unsweeetened form that calls for you to add your own sugar. Look in the powdered drink section, usually near the juices in the store. You can buy just one for very little money. I can't think of any way to substitute for it as it has the tart citrus flavor that you need.