I have been making my own buttermilk for about a year now maybe a little longer. I can tell you the quality of homemade buttermilk far surpasses anything you can buy at the grocery store.
Buttermilk is very easy to make. It takes bit of time to set but it's well worth it in my honest opinion.
I purchased buttermilk starters from here: http://www.leeners.com/ are a great company to work with.
You will need:
1 qrt canning jar with lid and ring
making my own buttermilk....
Fill at 1 quart canning jar with whole milk,I then pour that into a saucepan and warm it to 72 degrees. Then pour the warmed milk back into my canning jar and add one packet of buttermilk starter. Seal the jar by putting a canning lid and ring on it and tighten it. Give it a good shake and then set it on your stove top for 12 to 24 hours.
I set it on my stove top to keep the milk at 72 degrees or as close as I can to that. If it doesn't the milk won't set. I can tell you that mine sets closer to 12 hours than 24.
You will notice it's done when the milk is nice and thick. I use alot of buttermilk in my baking and have found homemade buttermilk is far better than store bought.
To make more simply take 3 tablespoons of my homemade buttermilk and use that as my starter instead of the packet of starter.
The Happy Homemaker