The best place to start is the Yorkshire Terrier Club (YTCA) website where they have a list of recommended breeders. This will guarantee that the breeder is following the YTCA's rules about breeding to the standard, health testing, etc.
http://www.ytca.org/breeder1.html
Any of these breeders will probably be able to refer you to other great breeders if they don't have any litters expected, etc.