Originally buttermilk was the liquid left over from churning milk to produce butter. All the fat was in the butter so it was essentially low fat milk.
Traditional Southern buttermilk is "cultured" with Lactobacillus bacteria, either starting with low fat or whole milk. It contains the live bacteria culture along with probiotics, just like yogurt.
The bacteria produces acid, giving a tart taste and thickening the protein in the milk. Unfortunately many producers feel the need to "enhance" the flavor and texture by adding junk ingredients like gums & flavor additives.
You should be able to find an organic buttermilk at better grocery stores. Beware that many may start with organic milk and add stuff to it, and still label the result organic.
If you make your own by adding vinegar or lemon juice to regular milk, you will get the tartness and thickness but not the complex taste and active cultures.
If you want to make your own with lactobacillus, start with some organic buttermilk and
USE THIS METHOD.
You can make any amount. Just keep some of the previous batch to make the next batch, kind of like sourdough bread.
You can also buy lactobacillus powder at health food stores and start with that, although it would be trickier.