Good news! You don't have to because you can easily make your own buttermilk if you don't have any (or don't want to buy for just one recipe). The ratio is 1 tsp. of acid per 1 cup of milk. For this recipe, I recommend adding 1.5 teaspoons of lemon juice to 1.5 cups of whole or 2% milk, but you can use reduced fat or skim milk too.