Ascorbic acid (vitamin C) is used in the food industry as an antioxidant, nutrient, color stabilize in cereals, fruit drinks and cured meats. Ascorbic acid is used to maintain the red color in cured meats and is added to some beverages to increase the vitamin C content. Much of the ascorbic acid used in food production is derived from corn. Ascorbic acid helps maintain the red color of cured meat and prevents the formation of nitrosamines, which promote cancer (see sodium nitrite). Vitamin C is also used to pump up the vitamin content of foods like "fruit" drinks. It helps prevent loss of color and flavor by reacting with unwanted oxygen. It is used as a nutrient additive in drinks and breakfast cereals. Sodium ascorbate is a more soluble form of ascorbic acid. Large amounts of ascorbic acid may slightly reduce the severity of colds.
You should be able to find this in your local health food store or a well-stocked grocery store. Walmart typically carries it. You can also purchase it online at Amazon.com from The Berry Farm: Ascorbic Acid