There is some information on U.S. pricing here:
Alpine Wheel Dealer! - Ducati.ms - The Ultimate Ducati Forum
This U.S. firm has Alpina wheels but doesn't list Ducati bikes:
Alpina Wheels
This U.K. outfit seems to sell Alpina wheels for supermotos, might be worth contacting them to see if they can offer the size you need for your PS:
Motobits.net - For all your biking needs
Perhaps these people may be able to help? They are in London:
Corsa Italiana Ltd. www.apriliasupermoto.com
Good luck with your search & keep us posted.
