Could sour cream be used instead of buttermilk?

Yes, but you'd have to thin it with milk to provide for some of the liquid. I would suggest using just about 1/3 cup of sour cream, and make up the balance with milk to the 1/2 cup total called for. If it is full fat sour cream, you might also consider cutting out a tablespoon of the butter, because the sour cream will provide considerably more fat than buttermilk- which is usually a skim milk product. I would then make one test cake to see if you need to adjust the leaveners, because the sour cream could also make it a bit denser batter.