Where can I purchase duct board or 'board' to replace my existing duct work?

I am in the San Francisco Bay Area and cannot find what I am looking for. The existing duct work is 2.5 feet wide and 6 inches tall. I will be replacing 5-15 feet of duct work beneath a trailer. Can an average Joe buy duct board for a DIY project. It seems like a simple project, but I cannot find the materials locally or online.