The Yorkshire Terrier Club of America's breeder referral page is a good place to start: ytca.org.
To help you screen if you find a breeder not on that site: Reputable breeders will not place a pup before 12 wks of age. They need this time to socialize, also they are a very small breed prone to hypoglycemia (low blood sugars) in young pups. A reputable breeder will not advertise "teacups" - there is no such thing, marketing ploy. A reputable breeder will have parents on site to be viewed and will want to interview you to make sure you will provide a good home.