Checkout the Market Place section of this website for items from board members.
You might be best putting up your own "wanted" advert listing all the bits you need, as it might prompt people to sell any spares they have.
You can also check
http://www.ducatitrader.co.uk/ as they have a good listing of spares.
Finally you could try
http://www.ebay.co.uk/ but the prices on there tend to be a lot higher than anywhere else ... although still cheaper than Ducati direct!