I always recommend going to the YTCA referral site, they usually have breeders in every state and you can interview a few. I believe everyone should visit the breeder's home in person, and make sure they are keeping the dogs in a safe and humane way. By the way, are you planning to breed or are you just looking for a pet? Most YTCA breeders will not sell a breeding dog to someone they aren't mentoring and this means you have to champion the dog before you can breed. Pets are always sold on limited registration, and to me these are two of the signs of a good breeder.
Yorkshire Terrier Club of America About The Club Best of luck in your hunt.