Buttermilk is used for its taste as well as adding softness to the fried breads. But no you don't. Plain yogurt, thinned with a bit of milk, can be used as well as regular milk that has been soured. Just remove 1 tablespoon of milk and replace it with 1 tablespoon of lemon juice or white vinegar. Wait a few minutes until it becomes thicker and can separate slightly.