Most asian groceries have them in the frozen section. I've bought them before at A Grocery Food Warehouse [1487 West Sunset Boulevard, Los Angeles, CA 90026 (213) 482-4803)], and I've seen them at several Ranch 99 Markets (http://www.99ranch.com). Also try the Thai grocers in Hollywood.
Note that "A Grocery Food Warehouse" used to be "A1 Grocery Food Warehouse", but in the process of looking up their address, two web sources say that they have dropped the "1" from their name. Whatever it's name is now, it's a great little store.