There are several pioneer associations and groups in our state. Perhaps you already belong to one or more of them?
There is the Daughters of the Washington Pioneers. The Clark County Genealogical Society (Vancouver, WA) has two filing cabinets or their materials including membership applications. Membership in this group has declined and they are on the brink of big change, Lethene Parks, Librarian of CCGS told me. Her group has plans to work with the group and hopefully digitize and post online an index to those files.
There is the Pioneer Association of the State of Washington. Organized in 1871, this group is headquartered in the Fiske Library building and maintains its reference library there. Contact www.wapioneers.org for more information.
And there is the Sons and Daughters of the Oregon Pioneers, which, since Washington was part of Oregon Territory, this can include Washingtonians. To be eligible to join this group, your ancestor must have arrived into the Oregon Territory before statehood on 14 Feb 1859. (That’s 30 years before Washington statehood.) Google it for more information.
Lastly, WSGS offers Pioneer Certificates and First Citizen certificates and recognition. Click to www.wasgs.org that information.