First of all, let me commend you for doing your homework before making this commitment. If you read the sick/injured forum you'll know that many people have bought adorable puppies that later turn out to have genetic health problems. These can range from semi-serious yet costly, ($1,700 and up LP surgery one leg.) to very serious, and very costly. Many breeders breed dogs without even knowing whether or not they have a genetic illness, they do no health screening, but just ask the vet can I breed the this dog, who just checks the obvious, i.e. the dog isn't in it's death bed.
I always recommend people start by checking with the mother club of the Yorkshire Terrier the YTCA. The Yorkshire Terrier Club of America, sets the standard for the yorkie, and it's breeders have some of the highest ethics you will find. Lots of people think that since they themselves aren't planning to breed or show, they don't need a dog that is that good. However, YTCA breeders sell mostly to people that are not breeding and showing, and sell with something that's called "limited registration." This means that you are not allowed to register the offspring of the dog, should you breed. I would not buy from a breeder who did not offer limited registration, and this is one of the ways you can tell a quality breeder from a backyard breeder. Remember also, that since these breeders are not into the "business" of breeding. Their prices are extremely competitive and even lower than some breeders who are just better at marketing, and are in the business of breeding. Supporting a YTCA breeder, also means you are supporting high breeding standards and principals, and are doing something that is good for the breed. The downside of buying from a YTCA breeder is that they are more like a rescue and are very choosy when picking a home for the pups, if you don't meet their requirements, they won't allow you to adopt. You can learn a lot, by just visiting their website and they have a breeder referral list as well.
Here are some links I have found useful:
Yorkshire Terrier Club of America
Post 6
http://www.yorkietalk.com/forums/gen...ml#post1900263 http://www.hsus.org/web-files/PDF/Good_breeder.pdf
Good luck with your quest, I know you are excited, but finding a really quality breeder is so important, genetics are the primary indicator of future health.