My used Voyager/Hinterberg frame came with canvas leaders. It is different than duck. I believe canvas is better than duck because canvas can take getting wet better (cotton duck does *not* respond well to water!). I believe you can purchase canvas by the yard from art stores such as Dick Blick.
Edit: I found a link to canvas on a roll online. You would want the unprimed canvas:
http://www.dickblick.com/products/bl...kTracking=true