Saco Buttermilk Powder makes about 3.75 quarts of buttermilk and costs around $4! I found it at my local Piggly Wiggly grocery store for a little less than $4. When baking, you just mix the powder with your dry ingredients and add water where it calls for the buttermilk. I used it in my Mama's Red Velvet Cake and it worked perfectly!
4 tbsps powder = 1 cup buttermilk