You can definitely find it at the farmers market in Santa Monica or Hollywood. Try Flora Bella farms. I don't think purslane is in season now, however. I'm not entirely sure of the season for purslane, but I think it's summer.
It's hard to find at Farmer's Markets... SOMETIMES it shows up in Hollywood... but it's only that one organic vendor at the intersection. The best bet is at Mexican Supermarkets although I find the quality can REALLY vary...