Tahini is just ground sesame seeds. You can find it in grocery stores in the nut butter section. I find that grocery stores with a good natural section usually have the best options. You want a somewhat runny tahini, and not one that is solid and hard; it needs to be stirrable. I recommend that you do not buy Joyva brand of sesame tahini. This brand is commonly found in most stores, however, it is much too hard to stir into a recipe. You will notice that most brands of tahini will have a layer of oil at the top. Make sure you stir it well before measuring out what you need for this recipe.