Good for you for doing your research before buying your Yorkie. Too many people buy a puppy first, then find out after the fact that they didn't deal with a reputable breeder. If you read the posts here, we have a lot of Yorkies dealing with liver disease and luxating patellas, both genetic conditions in Yorkies that cost thousands of dollars to treat. Doing your homework now can save you big vet bills later on.
Missouri is a tough state because it is the so-called puppy mill capital. There are more puppy mills there than all the other states so you will have to be very, very careful. I would definitely recommend sticking with breeders recommended by the Yorkshire Terrier Club or other breeders they may refer you to.
Here's the list:
http://www.ytca.org/breeder4.html#MO