Where is the best place to search for recommendations on reputable dog breeders?

Although I have little experience with South California specifically, I do have some experience in finding experienced, reputable breeders. There are a lot of "common" and simple answers here. Facebook and Instagram are actually your friends. Many dog breeders are small business owners. They do not have a marketing department and limit themselves to what they know, many of them end up having only a Facebook page or group that they stick to. Facebook groups of the local area and pet-related groups for the local area can help you find good breeders.

Secondly, Googling several key phrases like "[insert dog breed] dog breeders near me", "local dog breeders", "[insert dog breed] near me", etc. can help you find pages for breeders near you. This isn't always fruitful but may enlighten you to other online avenues that your local area may use to communicate about dog sales. Some social media platforms are more popular per geographical location than others.

Lastly, and not always guaranteed to have a result, is ask local stores. Pet stores, especially small stores, sometimes know local breeders. Local breeders are likely to need pet supplies, make contacts, etc. Pet store employees and owners see many more dogs than most of us and are more in-tune with the local pet community. Even if they don't know a breeder for your specific dog breed, they can sometimes offer to keep an eye out. If that breed were to come in later, they may ask the owner if they know a good breeder in your area. As mentioned, this doesn't always give a result, but I have found local breeders, rescuers, groomers, etc. this way.

(European/German perspective - check how much this applies for you)

Over here, many reputable breeders are organized in dog breeder's associations/kennel clubs.

• Check kennel clubs for the features you're looking for. E.g. check out what activities they have with respect to the health of their breed, to ensure good breeding practice etc.
• Kennel clubs have breeder lists and also lists of matings and lists of litters.

Some questions that come to my mind, to kennel club and/or breeder:

• Do they keep track of health, character problems?
• Do they do/help/cooperate with research?
• What measures do they take to work for healthier dogs?
• Health & character checks for the dogs to be allowed for breeding?
• What measures to limit inbreeding/shrinking genetic population size (particularly if the breed you consider is rare)?
• Limits on breeding frequency, including for males?
• Do they encourage/prescribe trainings for their breeders?
• How much do they weight esthetic vs. character vs. working performance in their standards? Does that fit your needs?
  Sometimes there are different breeding lines (e.g. family retrievers vs. working dog) or even different kennel clubs.
• Over here, the kennel clubs have people ("breeding wardens") who advise the breeder but also go and inspect the kennel in general and each litter.
• Visit possible/encouraged/required (except of course the few weeks around litter date).
• At what age are you supposed to get the puppy?
• Will the puppy be de-wormed, chipped and have its first vaccination?
• Does the breeder ask you questions to make sure the dog fits and will have a good home?
• Particular contract clauses/things you have to promise?
  • Many reputable breeders here will put an option for them to take their dog back in case you cannot keep it.
  • I promised to do the X-rays required for breeding and in case he turns out to be admissible to have him entered in the kennel club's herdbook. Others may have to promise to neuter their dog. Both can be a sign of a reputable breeder.
• How experienced is the breeder?
• How well does the breeder's "environment" and in consequence the already begun socialization of the puppies fit your home?
  E.g. I'm living rural, among the reputable breeders with litters in a suitable time frame, I went with one in an equally rural setting so the pup already had 6 weeks of experience with rural outdoor life when I took him to his new home.
  This breeder, by the way, doesn't even have a web page other than being listed at the kennel club web page - but has full professional training as [agricultural] animal breeder.