I’m in eastern Washington so I don’t have personal experience gardening in that area, but this looks like a good resource. They might also want to check out WSU’s Yakima county extension office and the Yakima conservation district office- they have a plant sale in the spring. My conservation district’s plant sale has some hard to find plants for cheap, because they’re all provided through volunteer labor by master gardeners from their own gardens.

You’re right about variation across the state, Washington is probably the most ecologically/geographically diverse state in the US