You can make your own buttermilk. Simply put 1 Tablespoon white vinegar or lemon juice in a one cup measuring cup. Fill to one cup with milk and let stand for 5 minutes.
You can freeze the remainder. If you like to have small portions on hand for recipes like this, simply measure 2 tablespoons and place into ice cube trays. When frozen, remove to a covered container or freezer bag.
You can also freeze regular buttermilk.