I remember wandering round Sainsburys looking for tahini for use in making homemade hummus. It turns out making tahini is so simple - you just grind very lightly roasted sesame seeds with a little olive oil.
I haven't seen tahini for sale in the supermarkets here in Thailand. We can buy hummus but for some reason it is very expensive. But now we have it cracked - making both tahini and hummus is a doddle really.