Cows are sold through Mirabelle's shop in Sunshine Islands. You must first have a barn built by Gannon on your farm. Once you do, they will become available for purchase for 5,000G each. An adult cow can be milked using a milker every day. You can make yogurt, butter, and cheese with milk and sell them. Brushing cows with the Brush will increase their friendship and their milk quality. As you increase friendship points with your cow, the quality of milk will go up. If you win the cow contest, they will start to produce s-rank milk, and you'll be able to purchase a Jersey Cow.