Here are two examples of professional organizations organzing and preserving information about Circus History.
Special Collections Research Guide
The Lemieux Library has some specials collections you can use (by appointment only). Major subject areas:
Seattle Public Library staff can help you with your projects! They have the Seattle Room that include their Special Collections and staff available to help you.
Seattle Room Hours
Monday - Thursday: 11 a.m. - 6 p.m.
Saturday - Sunday: 1 p.m. - 5 p.m.
Some of the Special Collections are available online:
How to get a SPL Library Card - http://www.spl.org/using-the-library/get-started/free-library-service-areas