Food-grade citric acid, which may also be labeled as "sour salt," is available in stores such as Wal-Mart or other retail stores that sell canning supplies. Manufacturers such as The King Arthur Flour Company and Ball, a division of Hearthmark, LLC, produce food-grade citric acid for retail sale.
Food-grade citric acid can also be found in local and online health food stores and through chemical companies. Some vendors, such as Bulk Apothecary, sell in quantities ranging from 1-pound packages to 2,000-pound pallets. Food-grade citric acid is often added to fresh tomatoes during the canning process to preserve flavor. It is commonly used as an emulsifying agent for ice cream or to ripen cheeses such as mozzarella or ricotta.