Amazon offers the GoodSense Tussin DM Cough and Chest Congestion 6-oz. Bottle for $2.98. Order via Subscribe & Save to cut it to $2.83 and bag free shipping. That's about a buck less than you'd pay at local stores. It's recommended for ages 12 and up.
Be sure to cancel Subscribe & Save once your order ships to avoid being billed for subsequent shipments.