Tahini is a thick paste of ground sesame seeds, and is very popular in Middle Eastern and Asian cooking and, apparently, protein-enhanced oatmeals, as well. The Middle Eastern version is ivory in color and a little thinner than peanut butter. The Asian variety is a darker brown and has a stronger taste. The lighter tahini is considered to be the tahini of choice.
It should be available in the ethnic foods section of any well-stocked supermarket, as well as in any Middle Eastern market, some gourmet shops, and, as you have already found out, many online sources. We happen to like online shopping….