Do not buy online. You can find someone that way but you must talk to them, go visit them etc. You can start with the YTCA website, there is a breeder tab on it,
Yorkshire Terrier Club of America. If you can, contact a few of them somewhere close to you and if not in your area, ask them if they know someone who is.
When you contact them if you email first, tell them what you are looking for, a bit about the home you can offer. We are a very conservative bunch don't use text messaging type of short abbreviations. Give a phone number or ask when you can phone them.
Bear in mind at this time of year there are a lot of shows, people are busy with family, visiting relatives vacations, you may have contact a few times. Then there is me who loses emails and phone messages without remembering I was contacted.
As we do not breed to fill a pet market, often a reputable breeder will not have something readily available.
There should be genetic health contracts, they should be asking you many questions and be willing to answer yours. Most areas for a pet is about $1500 - $2,000 depending on where you are More than that is rather high for a pet.