As Garth indicates NBUA is one option if you are planning on living and working inside King County. However, there are other associations out there and it depends on the level of ball your are planning on doing.
If you are planning on High School Ball, you may wish to find an association that is within the area you either work or live.
A great source of information is the Washington Officials Association:
http://woaofficial.web123.discountas...s/default.aspx
From here you can select the sport, and area. It will give you the contact information for multiple associations within the entire state.