The washingtonlawhelp.org has recently added a new interactive interview to create a letter to a landlord for return of a security deposit.  “This online interview provides a completed demand letter for return of a rental security deposit in Washington State. Use this program if you did not receive or only partially received a refund of your rental security deposit from a previous landlord and wish to send a demand letter requesting that it be returned to you.” 

They also have one of their standard kits of instructions and forms to do the same thing.