well you could check your local motorcycle shop or motorcycle gear shop. or....you could check out an online shop, that i don't know sorry, but i'm sure most of the other members will know.
this will show you how to do the dual visor: http://405th.com/forums/index.php?showtopic=2021
and as previously mentioned, you can get the visor from any motorbike shop, the above topic also has a link to a site you can get them