Many grocery stores and supermarkets such as Wal-Mart sell buttermilk. It can also be purchased from dairies. Buttermilk can produced either by aging or through culturing.
The process of aging buttermilk includes ripening and then churning fresh sweet cream. Culturing occurs when cultures are added to milk. If a cook is out of buttermilk, she can add lemon juice or vinegar to regular milk for use in a recipe, but the resulting ingredient doesn't have the same flavor and taste as real buttermilk.
To make buttermilk, combine 3 cups of heavy cream that hasn't gone through an pasturization or homogenization process with 1/4 cup of cultured buttermilk containing live cultures. Allow the mixture to sit for up to 24 hours, and then chill it for several more hours. Also chill a food processor, metal mixing bowls and a wire-mesh strainer. Pour the mixture into the chilled food processor, and mix it until butter forms and the buttermilk separates.
Pour the mixture through the cold strainer to separate the butter and buttermilk into the chilled bowl. Chill the buttermilk. Combine 2 to 3 cups of ice water with the butter, work it with a spoon until it forms a solid mass, and refrigerate it.