so plain yogurt makes for a good substitute. You can replace buttermilk cup for cup with plain yogurt, but it may work better to thin the yogurt with water or milk, especially for recipes that ...
if you don't have a ready supply of buttermilk, you can make a quick substitution for buttermilk by souring some fresh milk. Add 1 tbsp of lemon juice to every 100ml of milk called for in the recipe.