First off...Welcome to the forum!!!
Secondly...the upper 4 counties in Washington have ghost towns all through them, though the wife and I have only been through Okanogan County ourselves for the past 3 trips and still haven't seen it all. It has the most in ghost towns, we think. Also, check Ferry, Stevens, and Pend Orielle Counties on the web site map.
http://www.ghosttowns.com/ (Click on the "USA/Canada Map" and then click on "Washington State".)
This may help too.
http://www.ghosttownsusa.com
Bad Bob has some GREAT info.
Enjoy your trip and have fun!!!