You can purchase Sheep from Jessica at her shop in Bluebell for 7,000G. With the purchase of your first sheep, Jessica gives you shears so that you can harvest the sheep's wool. Sheep can be sheared every 4 days and will produce wool. The higher your friendship with a sheep, the better the quality of the wool. Wool is quite expensive, and sells for more than milk or eggs.
Sheep have a long life span, averaging about 7 years.