Here is an idea to help you in a pinch: I ran out of sour cream that I use in the cake mix extender recipe I use from this site. I didn't have yogurt either and was in a panic. What do I do?? I was doing 2 box mixes with 1 extender recipe, so I pulled some cream cheese out of my fridge and let it soften a little. I put about 4oz in a bowl and beat it with some milk to give it the consistency of sour cream. I put in what I thought was the comparable amount and baked it. I couldn't tell the difference!

Thanks for the tip. I am always running out of something!

wow!! thank you for the tip