BENTONVILLE, Ark. -- Wal-Mart has joined forces with Horizon Organic, one of the country's leading organic dairy manufacturers, to supply selected stores with organic milk in the dairy case. The Boulder, Colo.-based supplier began shipping half gallons of milk to certain Wal-Mart stores late last year, said Clark Driftmier, senior vice president of sales and marketing for Horizon. The company's products can now be found in about 45% of supermarkets, and virtually all natural food stores, Driftmier said. Founded in 1991, the company produces organic milk, eggs, juices and other dairy items.