I buy from a pre-order farm market, Clemson Area Food Exchange. For a while I bought goat milk yogurt there, but learning about a primal diet I kept reading about the advantages of raw milk (and the goat milk yogurt is pasturized). CAFE offers raw milk from Jersey cows, which some people think has a healthier kind of protein than the more common milk from Holsteins. I felt a bit intimidated by the raw milk both because there is so much written about the risks and because I remember not liking it as a child, so I decided to make yogurt to use in my smoothies.
After some research, I decided that I wanted to keep the advantages of the raw milk and not heat the milk to 180 degrees, as most recipes call for. I'm currently using purchased yogurt starter. If you use raw milk you shouldn't just start your yogurt from the last batch, as other bacteria can increase. I didn't want to go to the trouble of making separate batches in pasturized milk to use as starter, and the purchased starter has the advantage of a wider range of live cultures than even good supermarket yogurt.
So here's the system I have worked out. To my surprise, it makes a nice firm yogurt even though it is low fat and from raw milk, both of which tend to make yogurt set less well. I thought about buying a yogurt maker but I keep reading reviews that they run too hot or too cold so I decided that the cooler method would work as well.
Raw milk yogurt
Sterilize glass canning jars in boiling water (I make two quarts). I pour off most of the cream from my half gallon of raw milk to use for other purposes. Fill your jars (leaving at least an inch headspace) with milk and place in the pan of warm water left over from sterilizing the jars. Let sit, stirring occasionally, until it reaches 115 degrees. Stir in yogurt starter. Cover loosely (I use the plastic storage lids sold for canning jars). I wrap the jars in a towel inside a cooler with a hot water bottle of hot tap water and a couple of insulated travel mugs of boiling water and let it sit on the counter for 8 to 10 hours. Refrigerate.