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For recipes, a substitute for buttermilk can be made by adding 1 tablespoon of lemon juice or vinegar or 1 3/4 teaspoons cream of tartar to each cup of regular milk. The soured milk should be allowed to sit for ten minutes before being used.[4]

A low-fat substitute can be made by mixing equal parts of skim milk and low-fat yogurt.[5]
For further reading see the Wikipedia entry here.