Ordinarily, I hate revealing inside info to gringos, but this board has been good to me.
New Capital Seafood Restaurant in Arcadia (855 South Baldwin, just below Huntington Drive, next to KFC), is currently offering a special on steamed jumbo prawns, 99 cents/lb, with a maximum order of four pounds per table. It's only available on weekdays from 3:30 to 9:30. Don't know how long this will last, so you should probably call before making a long drive: (626) 445-9998.
WARNING: Be prepared to wait (Reservations? We don't need no stinking reservations!), and your odds of finding a space in the parking lot are poor.