I have a favorite muffin recipe that requires buttermilk. I typically don't keep milk in the house at all because I think it slows mucous functionality. However, I'm willing to buy some for my favorite muffins. If you need milk for some other delicious recipe, it might be best to just make your own homemade buttermilk.
Very simple recipe for this one.
1c regular milk
1 tbsp. white vinegar
Simply mix the two together and let sit for about 5 minutes or so.