As internet sites advertising litters go, Champdogs is one of the better ones...if they say the parents are health tested then they've had to provide verification of that and to sell at all then they've had to give KC numbers and pedigrees for the parents, which means you can view (and possibly research lines) the pedigrees before you ever get in touch with the breeder.
I'd look on the KC lists and Champdogs at breeders, but, obviously you still need to do your research on health testing and follow all the normal puppy buying advice as well. It is a better starting point than other websites though.
I second the Black Retriever Rescue people, but if you do insist of getting a pup from a breeder, rather than a rescue, you would be better off choosing a breeder who is on the Kennel Club's Assured Breeder list.
Anyone can register their dogs with the Kennel Club, effectively making them 'pedigree' but that doesn't make them a good breeder. Assured breeders have to adhere to certain conditions and restrictions that are there to protect the bitch and puppies, as well as going a long way towards making sure you get a healthy, well socialised pup, rather than one who has come from a backyard breeder and may have all manner of health problems and a poor start in life.
Also, please investigate which health conditions are prevalent in the breed and what tests the parents should have had eg, eyes, hips etc. The Kennel Club Assured Breeder programme will help you choose a breeder who has made sure both dam and sire have had all the checks, but you also need to be aware of which test results are necessary for them to produce healthy pups.