Do you know the name of the court that issued the parenting plan? Do you know the case number? (The case number is on the upper right corner of the case caption on the first page of each document filed with the court.)
If you have those pieces of information, you can contact the court for a copy of the document. If the courthouse is near where you live, you also have the option of going to the courthouse and make copies.
The clerk of the court will charge you money to make copies or for you to make copies. Certified copies cost more but will be accepted as true copies by other places. If you do not have the case number, the clerk may charge you a research fee.
To find your court and case number, you can run a "name search" here: http://dw.courts.wa.gov/index.cfm?fa=home.home . Click on "courts home" and you will find links to get the contact information for all courts in WA.