The Yorkshire Terrier Club of America website has a list of breeders who have agreed to its code of conduct and ethics. You can easily search for breeders within your own state using their interactive map. The American Kennel Club's website also lets you find Yorkshire clubs by ZIP code.
The Yorkshire Terrier National Rescue has Yorkies available for adoption as well. Many of its dogs are older or have special needs, so those who wish to adopt a dog should allow for the additional expenses and time they might require.
Another adoption possibility is to use PetFinder. Search for Yorkshire terriers available for adoption nearby and contact the owner. It is riskier, naturally, to adopt from a stranger over the Internet as opposed to from an actual rescue organization, but many pet owners have had wonderful experiences using PetFinder.
Those who decide to go with a breeder should do their homework. Research several breeders, visit them to see the conditions the dogs live in. If possible, find people who have purchased from that breeder in the past and find out if they are happy with their dogs. A local veterinary clinic may also be able to recommend a breeder.