I make my own yogurt, and then I strain it to make Greek yogurt. I just put a basket coffee filter in a colander over a bowl in the fridge for about 5 hours or overnight and you've got Greek yogurt. When we were doing a kitchen remodel I would just buy plain store-bought yogurt and strain it. You can buy yogurt culture online and I usually will make my next batch using a couple tablespoons from the last batch. I reculture every other batch. I'm sure I could go longer, but I use fresh milk from the farm and would hate to waste it on a batch that didn't turn out.