Recipe(tried): How to Make Sour Milk and Substitutes for buttermilk

Generally, old-fashioned recipes will specify "sweet milk" when regular, fresh milk is meant, and "sour milk" when buttermilk is required.
You should be able to successfully and simply substitute for the buttermilk, if you care to, by making your own sour milk:

Mix 2/3 cup plain nonfat or low-fat yogurt plus 1/3 cup milk... OR

Place 1 tablespoon freshly squeezed lemon juice or distilled white vinegar in a glass measuring cup. Add in milk (2% or richer) to equal 1 cup. Stir. Let stand for 15 minutes until it looks like its beginning to curdle or slightly curdled.