What can I use to substitute buttermilk in a cake?

I know this isn't exactly a substitute but if I run out of buttermilk I usually just use whatever buttermilk is left and then use whole milk for the remaining quantity. Additionally, I have changed recipes from whole milk to buttermilk with no other changes to make my cakes and cupcakes more moist. Cake recipes are often forgiving and I love to experiment! If I were in your shoes, I would use whole milk and hope for a happy discovery!

But hey, I'm also the kind of girl who doesn't have a problem with some sort of plan B like cakeballs in case of mishap..... I know not everyone is like this.

You can substitute buttermilk with milk, but when you do you must be aware of the acidity of buttermilk that you have to neutralise by adding baking soda. This in turn will add leavening power, which according to W. Gisslen, you compensate for the as follows:

US: For each quart buttermilk ADD 0.5 oz of baking soda and SUBTRACT 1oz of baking powder
EU: For each liter (1kg) buttermilk ADD 15g baking soda and SUBTRACT 30g baking powder

So to summarize: substitute the amount of buttermilk with the milk but adjust measurements for baking SODA and baking POWDER as per formula above.

You can also use two tablespoons lemon juice in a cup of milk.

1 Tablespoon of white Vinegar and fill to the one cup line with Whole Milk. Let stand for five minutes. I heard about this before, but I grabbed the measurements from About.com (I wasn't sure how much vinegar to milk)