Part of the TAT
Have you looked at the Trans America Trail route to see where you could pick that up. If my memory serves correct, you should be able to head northeast to Nevada and then pick it up there and take it east. I don't know how close that will go by the town you need, but it might make for a nice ride. All of it is doable on a dual sport, but as you probably know, the lighter the more fun, especially in the silty sand portions of Nevada and Arizona.
Let us know which way you end up going and good luck with your search.
P.S. What bike are you going to use.
125 Honda (first bike at 13 years old), Early 70's 250WR Husqvarna, Late 70's 390 Husqvarna, Honda 1000VFR (stupid fast ), 2007 BMW R1200RT, 2008 KTM 530 EXCR