I would never buy a puppy on-line; this is one of the main ways puppy mills sell puppies. I think the best site for finding ethical breeders is the YTCA.
Yorkshire Terrier Club of America About The Club The human society has a pamphlet on finding a good dog.
http://www.hsus.org/web-files/PDF/Good_breeder.pdf , and YorkieTalk had a recent thread about this very topic, and one of our members thought up a very good list of questions.
http://www.yorkietalk.com/forums/gen...ml#post1900263, but you may need to do more reading before you understand what the right answers are. Don't take shortcuts, hopefully this pet will be with you at least 10-15 years and you want to make a wise choice.