I haven’t seen one of those used online and if there was it would be a ton for the oddly shaped box + shipping. I picked mine up from Comet a few years ago and was at my door in a few days (I was in a rush to get one also).
http://cometkartsales.com/Righetti-Plastic-Rear-Bumper-Black.html
take your pick on OEM Homologated or aftermarket hardware
http://cometkartsales.com/Rear-Bumper-Hardware/
Sean