Never, ever buy a puppy from anyone who advertises them. Reputable breeders never advertise as they'll have waiting lists for puppies.
Go to dog shows. As many as you can. Get a copy of the schedule and look up the KC names of the dogs you like the look of. While you're at shows, talk to people. Owners/handlers. Many of them will be very experienced in the breed and can advise. Visit Discover Dogs at Crufts.
Contact the Kennel Club and the official breed club of the UK. They can provide a list of reputable breeders. Champdogs is OK but used by puppy farmers who sell KC reg puppies.
When you find a breeder you like, make sure that the puppy will come with a contract that includes endorsements and a clause that will allow you to return the dog to the breeder at any stage in its life, if it comes to that. Ensure that both dam and sire have the relevant health tests for the breed, and you see those certificates/scores. Make sure you see the dam with the puppies. You don't need to see the sire usually - many breeders will use a stud from a different kennel if it betters the breed.
Finally, be prepared to wait. Good breeders will have people waiting for puppies. Be wary of anyone who has puppies available immediately. Ask yourself why. I waited five years for one of my current dogs. So worth it.
Oh, and research LOTS. Good breeders will want to know that you are familiar with their breed, know what you're getting into and will usually ask lots of questions.