You can use regular yogurt. Make sure you get plain and NOT a low fat version. Line a small strainer with coffee filters (or cheesecloth), pour the yogurt into it, cover with more coffee filters. Place the strainer over a bowl, and set a pot with water in it on top of the yogurt, inside the strainer. That will help squeeze out the liquid and make it thicker like a Greek Yogurt. Put it in your fridge for a few hours while the liquid drains and then take the yogurt out of the coffee filters, put in a bowl and proceed as you normally would with your recipe.