A recipe for a buttermilk substitute is available at TheKitchn.com and uses just two ingredients: milk and white vinegar or lemon juice. The whole process takes five minutes.
To make a buttermilk substitute at home, combine 1 cup of milk with 1 tablespoon of white vinegar or lemon juice. Give the mixture a quick, gentle stir, then let it sit for five minutes. After the five minutes have passed, you have buttermilk for your desired use.
Note: you can use either whole milk or 2% milk for this recipe, but whole milk will give a thicker consistency more closely resembling the buttermilk available in stores.