What is tahini, where can I find...

Tahini is sesame seed paste and is used primarily in Middle Eastern cooking. You should be able to find it in the international section of a large grocery store. You can also make it yourself with a large amount of toasted sesame seeds and some oil. Put the toasted seeds into a food processor and stream in the oil until you achieve a desirable consistency. A close substitute in flavor is peanut butter, so you might try experimenting with that as well.