While the taste of buttermilk can't be beat, sometimes you run out or just need an alternative. If you need a substitute, you can make sour milk. It’s a good substitute and very easy to make.  Mix a cup of milk and a tablespoon of lemon juice or vinegar and allow it to sit for 5 minutes. This will curdle the milk and make the best substitute that can be used in place of buttermilk.