In my opinion the MOST important thing to consider when buying a puppy is the reputation of the breeder. All Yorkie puppies are cute and most seem healthy when you look at them. The only way you can get a better idea about what is going on under that skin is learning all that you can about the parents of the pup and the breeding practices of the breeder.
I wouldn't ever buy from Internet Ad, wouldn't ever buy from the newspaper and I wouldn't ever buy from a petshop. If I wanted to buy from a small hobby breeder it would only be after someone refered them...someone who had a dog from them that was a couple of years old and healthy.
Otherwise I would go to the YTCA website and contact the breeders on there and get on a waiting list.
Don't be in a rush! These dogs are wonderful but owning one can be heartbreaking if you don't do your research. They are so popular right now that a lot of unethical people are cashing in on it.
www.AKC.org www.ytca.org
Good luck