Wow...69 views and no posts...LOL.
Try here: http://www.discountramps.com/carriers/c/2130/
If you have a local classifieds or Craig's list you might find a welder that makes them - in many cases they're just as good or better than the factory ones and usually a lot cheaper.
$300 is quite a bit for one, not unusual to find a used one or welding works one for even less.
I recommend an integrated ramp, or way to store it. You'll need one to load the moped on the rack.