"The staff of the Idaho State Historical Society, a number of intrepid volunteers, and members of the public from around the state have identified over one thousand Civil War veterans who lived or died in Idaho. These records are indexed alphabetically by surname, and whenever possible include dates of births and deaths, the states from which they served, and the Idaho counties where they resided. For many veterans, more than one record has been uncovered."
If you have ancestors from Eastern Washington, it may be that they lived at one time in Idaho. This is certainly a site worth checking.