When you are baking, strictly sticking to a recipe WRT ingredients or amounts is important. With other cooking, it is not as critical. The purpose is too add a little silkiness and creaminess so yes, just pick up a small carton of cream, milk, or half/half.
BTW--as far as buttermilk, I rarely see more than one or two brands even in the big groceries so I don't think your store is unusual. I never can use a whole carton so I freeze leftover buttermilk in baggies.
Post a pic...the dish sounds really good!!