The FSBO section of this site often has good deals on lightly-used boats. (Full disclosure: I have one listed there currently.)
Aqua Sports in Redmond is a great local resource for WW boats and gear. Northwest Outdoor Center on Westlake in Seattle often has some used whitewater boats and carries a mix of WW and ocean gear. Leavenworth Mountain Sports, Kayak Shed in Hood River, and Alder Creek in Portland are good regional options.