Once you decide on a breed or breeds you like, I think then is the time to look for a breeder you like. It especially important to look for good breeders when purchasing a "popular" breed. Many people are breeding Yorkies not to improve the breed, but just as a quick way to make some cash, you want to know what kinds of health screening the breeder does on the pup's parents. Most backyard breeder do no health screening. Here's a link to finding a good breeder.
http://www.hsus.org/web-files/PDF/Good_breeder.pdf. This thread has informtion on questions to ask.
http://www.yorkietalk.com/forums/gen...ml#post1900263 Don't be in a hurry this is a 10-15 year commitment. It is good that the breeder waited till 12 weeks to sell the puppies, many don't, and 12 weeks is the minimum given by the YTCA.
I just want to add that a healthy Yorkie is as healthy as any breed.