What do I substitute for "sour milk"?
You can use regular milk and vinegar. 1 cup milk to 1 teaspoon vinegar. I usually use cider vinegar.
I use a tablespoon, not a teaspoon, and sometimes use lemon juice instead of vinegar, if that's more convenient. I typically put the vinegar in the measuring cup and then pour in the milk to make a cup or up to 1 and 1/4 cup (whatever the recipe calls for). I do this to make my own buttermilk in a pinch, too, but only for buttermilk used in baking, and not for other types of recipes.