A good place to start is the website for the Pembroke Welsh Corgi Club of America. They have a breeder referral look up for each State and I believe links to recognized clubs in various States so those website would be worth pursuing also. There is an active PWC club in Southern California although their website seems to be down tonight.
Cooper is from Poso Creek Puppies up in Bakersfield. I would definitely recommend this breeder. I know some of his relatives, and they all have a very good temperament, and are super adorable!
Here is the website: http://posocreekpuppies.com/