This is taken from the Yorkshire Terrier Club of America:
http://www.ytca.org/faq.html
"Parti-Color Yorkshire Terriers?
Do not be fooled into buying one of these dogs.
Now that Designer Dogs are the rage, the “new” Parti-Color Yorkie is certain to draw attention. While we have had problems in the past with “rare gold” Yorkshire Terriers being advertised, the parti-color is a new one! While some breeds have an occasional mismark and some breeds do have a gene for a white dog, we do not. Had there been a problem with white markings, piebald dogs, or white dogs, it would have been addressed in our Standard. Due to unscrupulous breeders advertising parti-colored Yorkshire Terriers at premium prices, our members voted unanimously at our annual meeting to add a disqualification for these and other off colored dogs."
In other words, the American Kennel Club does not accept them; so ethical breeders aren't selling them.