I'm baking a white cake & the recipe calls for 1/2 cup sour cream or plain yogurt, but I don't have either. What can I use instead?

Buttermilk will give it an amazing and smooth texture. You couldn't ask for a better substitute..

It's the tang of lactobacillus that you want to reproduce as well as the fat/protein content of the dairy. Buttermilk has the same cultures as yogurt or sour cream so the flavor would work. The rest comes down to texture from losing the fat (from sour cream) or protein (from yogurt.) If you're worried about the loss of fat you could probably substitute buttermilk with butter and be fine.