I would substitute with plain non fat yogurt.... It works! Pour a slash of milk in you need more liquid.

Add one tablespoon of lemon juice to a liquid measuring cup. Then, add milk until the amount reaches one cup. Let the mixture stand at room temperature for 5-10 minutes. The milk should look curdled. Stir and you have homemade buttermilk or measure 1 3/4 tablespoon cream of tartar plus 1 cup milk. Whisk to combine and let stand at room temperature for 5-10 minutes, until curdled and then stir.