I am making homemade buttermilk this week from my raw milk. I let my starter go, so I'm starting over from scratch. I like to keep buttermilk on hand for pancakes, biscuits, etc. Plus it's a great way to keep from wasting my raw milk. Once it's cultured, it lasts a long time in the fridge.
You can use store-bought buttermilk as a starter, but I found that it only works well for a handful of times. When I start it from scratch myself, it lasts longer (the culture, that is).
Click on this link to see just how easy it is to make buttermilk at home.
If you need more information, check out this link.
Also, you can use this same method to make sour cream. Once you have a starter, either store-bought or homemade, use it to culture your raw cream. If you want it ultra-thick, you can strain some of the whey out (I use a coffee filter, basket-style).
The benefit of both these products, is that they are live foods, without any added junk. They contain necessary probiotics, and even if you are using the buttermilk to bake with, there is evidence that dead probiotics still benefit us (the heat will kill the probiotics)!
Other articles on live foods that you may enjoy:
Why I will ONLY drink raw milk!