If thee needs anything and cannot find it, just come to me and I'll tell thee how to get along without it. ~Etta Macy (quoted by James Alexander Thom in "Indiana's Self-Reliant Uplanders," National Geographic Magazine, March 1976).
I just typed up a recipe that included sour milk among the ingredients. Of course, you can substitute buttermilk for sour milk, or you can make your own, by adding a spoonful of lemon juice or white vinegar to sweet milk and letting it stand for a few minutes to sour.
Experienced cooks learn these tricks over time. If you would like to see a nice collection of food substitutions in recipes, please check out the About.com: Home Cooking page on Recipe Substitutions.