This is a recipe I have been using for years to make buttermilk in large quantities. I like to use it for baking as well as drinking. It makes pancakes, waffles, cakes, and the like rise very well.
 
 

BUTTERMILK RECIPE: 1 part buttermilk with 4 parts whole milk in a very clean container, mix, cover, and let sit overnight in warm spot (such as on top of a refrigerator), at least 12 hours. Keep in a refrigerator until needed. ( I have made this for years for an annual waffle breakfast I serve at Clermont College, serving about 120 people, with 1 quuart buttermilk plus 1 gallon of fresh milk, mix, and pour back into the original containers.)  It may be that less starter is needed, but it has worked so well that I have not wanted to mess with the proportions.

See the page on milk fermenting bacteria for a demonstration and discussion of Streptococcus lactis, which is the bacterium which performs this fermentation.