To find Yorkshire terrier breeders, try attending a local dog show. Also, the Yorkshire Terrier Club of America has a referral list of member breeders who agree to abide by the organization's code of ethics and conduct.
Attending a local dog show provides opportunities to meet and speak with multiple breeders and to ask questions about their dogs and breeding practices. Look for breeders with waiting lists, since that means they don't breed as often and may have more healthy, high-quality litters. Check that the dogs are AKC registered. Ask if dogs were screened for hereditary medical conditions or defects. Contact local vets to ask questions about breeders.
The Yorkshire Terrier Club of America has a referral list of member breeders who agree to follow ethical breeding practices. The code of conduct and ethics includes provisions about kennel conditions and medical screenings for adults and puppies. For best results, investigate beyond the referral as much as possible.