You all know I don't usually recommend a specific product, but after several missteps in making my own from live culture yogurt from my coop, I get my Greek Yogurt cultures from: bacillusbulgaricus.com
I heat whole milk in my crockpot to 190F, cool it to the indicated temperature, stir in the culture, then put it in my oven at 110F (yes my oven goes that low) overnight. Before my new oven, I would heat two microwave heating pads, place them under and over the crockpot and wrap it in a beach towel to sit overnight.
The next morning, I strain off some of the liquid, then put the yogurt in pint jars and into the refrig. One batch makes 6 pints after the liquid is poured off. If I am baking I will use the liquid in place of water or milk. If not, I mix in a little yogurt and my husband likes to drink it like kefir.
When I am ready to eat it, I mix with my favorite fruit, pumpkin seeds, honey, vanilla - whatever I want. This yogurt is so thick and creamy I also use it for sour cream, and in salad dressings, smoothies, pancake batters...
I sanitize the reusable jars in my dishwasher, but don't feel they need to boil them. The yogurt keeps for about 10 days.
The directions say you can use one cup of the previous batch to make the next one, and I do that once, but the 3rd batch starts with new culture again. I find if I reuse yogurt as the starter more than once, "wild bacteria" sneak in over time and alter the taste.