there is a middle eastern market (dunno the name) on artesia next door to the salvation army thrift store (south side of the street, west of hawthorne east of aviation)
normally i make my pilgrimage there to buy bread, but they offer a pretty good array of all sorts of other middle eastern food.
I bought some za'atar from Super King Market. It was on the wall of spices facing the produce area. But we warned.... it was stale and bland.
Instead I went to Penzey's in Santa Monica (they sell sumac too) and WOW. A punch in the tastebuds. Far superior to the stale supermarket stuff... the Torrance branch would be much closer. Also, Penzey's sells sumac as well, if you want to make your own as roz suggested.
Super King Market
2716 N San Fernando Rd, Los Angeles, CA 90065
This may not be your most cost-effective source, but you may want to check out Penzey's Spices in Torrance to see if they carry it.
21301 Hawthorne Blvd
Also, if you do find a middle eastern store that doesn't actually have the mix, find out if they at least have sumac. That way you can mix your own by adding thyme, sesame seeds and salt. I'm not sure of the proportions, I just play around with it until it looks and tastes good to me.