Above is the label from a sour cream container we go a while back... I now use this container to hold dry snacks for my children. As you can see, there is (probably GMO) cornstarch, guar gum and carageenan, among other things. Why, oh why do they need to ruin this usually yummy condiment?
For my homemade version, you will need about a quart organic cream (preferably raw) and 1/4 cup buttermilk. I bought the least gummed-up buttermilk I could get from the store, although it still had "annatto coloring" as the last ingredient. I do not own buttermilk culture, so this had to do.
Pour the cream into a glass storage container (a mason jar would work, also) and stir in the buttermilk. No heating required- just mix it together cold! Then, let the mixture sit on your kitchen counter for a full 24 hours, covered with a rubber band-secured cloth.