I'd contact the Okanogan County Gen. Soc., email@example.com. Most local Gen. Soc's have a library of resources.
Also the Okanogan County Historical Museum, 1410 Second N., Okanogan 98840, 509-422-4272 is another source of local info. They are open May thru Sept. and in the winter by appointment so I'd call or send them a letter. Sucessful Hunting!