Hi Cathy and welcome to our yorkie community.
The first thing that you'll hear from many people here is that there is no such thing as a teacup yorkie. Yorkie lovers do not use that term to describe yorkies. Yorkies do come in different sizes though.
The breed standard states that yorkies are usually between 4-7 pounds. Smaller yorkies are just smaller yorkies. There are health risks associated with smaller sizes.
Be sure to do your research so that you'll know what you're getting yourself into.
My Cooper is 7 months old and is 3.3# and Carter is 15 months and is 3.4#. Neither have had any difficulties to this point in their lives but that doesn't mean it will continue that way.
You might check out
http://mylittlemia.com. There is great information to be found about smaller yorkies on that site.
Good luck in your search! An informed buyer is a wise buyer!
Lisa