I work with the So Cal Golden Retriever Rescue and we use:
Cottage Pet Hospital
Dr. Sahai
900 E. Broadway
Anaheim, CA 92805
714-535-6714
They have been great to us. All the gals in the office are very kind and helpful. Since we foster rescue dogs that we snatch from the pound we have been there probably 10 time in the last year. They have treated everything from Kennel cough to Giardia for us. If they prescribe a med that is expensive and you can get it for less on Petmeds.com just let them know and the will usually match the price or be very close. You can tell them you were refereed by the rescue.
Good luck. I hope your pup will be OK.