Homemade buttermilk
When I was a kid, my dad cultured his own buttermilk which he used to drink. As a kid I couldn't stand to drink the stuff and I still can't stand drinking commercial buttermilk.
Today I also make my own buttermilk. It's pretty easy to make. It's pretty hard these days to find commercial buttermilk with live cultures you can use as a starter, but you can buy starter in powdered form which keeps in the freezer for a year or two. When you make it yourself you can control the thickness and acidity by how long you culture before refrigeration. This time of year my kitchen stays about 75-76F so it takes 8 hours to culture buttermilk the way I like it, which isn't as thick or acidic as commercial buttermilk. The taste is not unlike plain yogurt in drinkable form.
The same powdered culture is also useful for making homemade sour cream and cultured butter, both of which are far better than most anything you can buy commercially. Most commercial sour cream contains thickening agents and doesn't compare to homemade. Cultured butter is very hard to find commercially.
Once you make any of these things you can use part of the result as a starter to make any of the others, so one package of culture can go a very long ways.
http://www.cheesemaking.com/shop/buttermilk-ds-culture-5-pack.html
Edit to add:
If you want to duplicate commercial buttermilk, use 1% milk and culture to about the same thickness. Provided you heat the milk to 82F prior to pitching per the instructions, it takes about 10-12 hours of culturing in a 75-76F kitchen. If you want to vary from commercial buttermilk you can change things up by using 2% or whole milk which makes for a richer final product and you can alter the culture time to increase or decrease the viscosity. If you culture for too long, the whey will start to separate which you can skim off the top and the result will be very thick not unlike yogurt or sour cream.
= new reply since forum marked as read
I guess it worked okay... nobody knew the difference. 
