Buttermilk can be used as part of a substitute for sour cream when combined with other dairy ingredients. To substitute for sour cream in a baking recipe, About.com suggests mixing buttermilk with butter.
The actual ratio of buttermilk and butter in About.com's sour cream substitution recipe can vary depending on the consistency desired. To substitute for 1 cup of sour cream, either mix 7/8 cup of buttermilk with 3 tablespoons of butter, or mix 3/4 cup of buttermilk with 1/3 cup of butter.
About.com also suggests that 1/4 cup of buttermilk with 1 cup of cottage cheese, mixed briefly in a blender, can produce a substitute for 1 cup of sour cream for a dip recipe.