Surprisingly, you can make buttermilk substitutes, either dairy-based or nondairy, using ingredients you likely already have in your pantry or fridge. The key elements of a buttermilk substitute ...
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.
Dairy products go off rapidly in warm temperatures: milk, cream, yoghurt and buttermilk should be stored in the fridge, and cheese in an unheated room such as a larder, cellar or garage.