In California, the Phyto certs are handled by individual county Ag people, on behalf of the state Ag people, who are doing it on behalf of the USDA people. Getting and import permit for material coming into the Us is the same. You apply to USDA, they send it to the sate, and the state sends it to the county, and then the process reverses.
If Washington is similar, then you need to talk to a county person near where you live. Shipments into Washington also need a cert.