Yorkies can be great little dogs. I agree with Dr. Mark. They do have health issues. They also have personality issues that are genetic. One issue that comes to mine is luxating patellas. Make sure the sire and dam have been tested for luxating patellas and get those test results.
Unfortunately Yorkies can be aggressive. Be sure to "meet" the parents and see their temperaments. These little guys should be friendly, happy and pretty easy-going. Shy or aggressive lines should be avoided.
Remember, too. There are lots of dogs in shelters that are put to sleep every day. Unless there's a real need for you to get a Yorkie, I'd look at getting a small dog from a shelter instead. Then you get to save a life!!!
OR if you really MUST have a Yorkie (and I can't for the life of me come up with a reason you would need a "bred for purpose" Yorkie...), look into Yorkie breed rescue. Just Google "Yorkie breed rescue" and add in your state. From there, you can start sending out e-mails. Also, you can check petfinder.com and look for Yorkies in shelters in your area.
I, too, prefer well bred Malteses along with Havanese and Papillions when it comes to the small dogs. But a well-bred Yorkie can be a lovely dog, and I know one who is one of my all time favorite tiny dogs. :)