Finding a buttermilk substitute can be handy, especially when you're in the middle of a recipe. One simple swap is to combine 1 cup of milk with 1 tablespoon of lemon juice or white vinegar; let it sit for a few minutes until it thickens slightly. You can also use yogurt or sour cream diluted with a bit of water. What kind of recipe are you working on? It could help in determining the best substitute to use!