I recently bought tahini at Vons. It comes in a can (I think green?) in the "international foods" aisle. Trader Joe's also has a tahini dip (?) in the fridge section near the hummus, however the consistency is different than straight tahini.
There is a small Middle Eastern market on Garnet in Pacific Beach where I buy mine. I think it is named Ariel, and it is just west of Mission Bay Drive on the North side of the Garnet in a strip mall. They cary several brands. The one I use is in a jar.