Just order them from Honda. When I was making a trunk rack I ordered 6 of them (they went with the bike). FWIW, when I made the trunk rack I used some RTV to hold the darn things in place.
This is where you find the part. Part number 3, named the "Nut, Headlight Setting" Wonder where they got the name.
http://www.hondapartshouse.com/oemparts/a/hon/506c6245f870023420a39649/seat
PS the spacer for the tail piece should probably also be ordered (part no 20 on that page). But, they are the same as a collar sold by McMaster Carr for MUCH cheaper. I got a few from Katherine cause she bought a bunch but don't know if she has any more or not.