Hi Eileen W
This page from the WashingtonTrailsAssociation has a good explanation of the various passes. You're asking about the Discover Pass.
Here is the state info page about the Discover Pass http://www.discoverpass.wa.gov/
Hopefully a local can direct you to a store that sells fishing licenses and thus, the Discover Pass. Note that the Discover Pass is only used at State Parks and some Fishing Access parking lots. It is not used at county parks nor at National Parks.
You can buy your discover pass right at the state park. You can buy a day pass for $10 or an annual pass for $30.
A discover pass is not required if camping in the park. Your camping fees will cover the cost for that day.
Thank you for the reply and the links. Eileen