Are you sure you don't mean the one that goes inside the chair mount and outside the post? I have gone through several myself. My post also has a bushing that creates a plastic lip over the exposed metal edge of the post. That way the plastic ridge from the seat rides on the plastic rim.
our powercat has todd seat and plastic bushing goes around post but also has a cast in lip that rides on top of post. it's also available at boater's world.
btw, we had a great time in dc...thanks for the tips.