While visiting gardens in Tacoma, I had the opportunity to visit the Pacific Bonsai Museum. The museum has over 150 bonsai, with 60 that rotate in and out of display on a regular basis depending on the season. I found the collection to be very stunning. The outdoor environment of the museum also contributed to the appreciation of seeing the bonsai. Located on the old Weyerhaeuser campus, you will need your own transport to get to this museum.
Website: http://pacificbonsaimuseum.org