1. Buttermilk: This is a popular substitute for sour milk. It has a similar tangy flavor and consistency as sour milk.

2. Yogurt: Plain yogurt can also be used as a substitute for sour milk. Mix it with a little water to achieve a similar consistency as sour milk.

3. Vinegar or Lemon Juice: Adding a tablespoon of vinegar or lemon juice to a cup of milk can create a sour milk substitute. It will also thicken the milk slightly.

4. Cream of Tartar: A mixture of 1 cup of milk and one and a half teaspoons of cream of tartar can also be used as a substitute for sour milk.

5. Powdered Milk: Mixing a small amount of powdered milk with water can also be a substitute for sour milk.

When using these substitutes, make sure to adjust the measurements according to the recipe's requirements to ensure the desired taste and texture.

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.

You can use regular milk and vinegar. 1 cup milk to 1 teaspoon vinegar. I usually use cider vinegar.