I've bought them at Hawaii Supermarket, but I know I've seen them at the Pasadena Farmers' Market on Saturdays. There's a vendor that specializes in all kinds of herbs (I don't know their name, alas), and they've had them recently. I'm trying to remember where else I saw them in the area...could it have possibly been Whole Foods or HOWS?
They often have them at the Hawaii Supermarket at Valley and Del Mar - which seems to have more fresh southeast Asian produce than Ranch 99. The Bangkok market on Melrose near Western almost always has them. But they are surprisingly difficult to find. If you can't find them fresh, the frozen ones retain most of their flavor. I gave up and planted a kaffir lime tree in my backyard. Now I've got them all the time.