Welcome to the forum mate, it's called a rear wheel nut. Not sure where you are in the world, there might be someone nearby that could help you out.
It's a 55mm tapered nut and used on all the MV's, not just the Rivale, most of the forum Vendors in here - http://www.mvagusta.net/forum/forumdisplay.php?f=35 sell them but they are widely available. Do nut use a regular multipoint socket as this will damage the nut. Remember it's a Left Hand thread when you undo it.