The first place to look for Washington legislative history is online, at: http://leg.wa.gov/History/Legislative/Pages/default.aspx
If you do not find what you are looking for on that site, your next step is to contact the Washington State Archives. (http://www.sos.wa.gov/archives/) The Archives’ research service is physically located at 1129 Washington St. SE, inOlympia; for mail, use the post office box address: PO Box 40238, Olympia, WA 98504. You can also call the researchers at: 360.586.1492
The Archives’ business hours are Monday through Friday, 8:30am to 4:30pm. When you call or write the researchers, have the year and bill number ready. You can find that information in the Washington State Session Laws, at: http://www.leg.wa.gov/CodeReviser/Pages/session_laws.aspx