I bought a factory windscreen 48710091A from Ducati North America a couple of years ago, but it is now listed as obsolete in their database. You might try Ducati UK or Ducati Italy to see if they still have any.
Ermax
http://www.ermax.fr/ made windscreens for the factory Ducati team in 1993. If you look at page 105 of the first edition of Carl Fogarty The Complete Racer by Julian Ryder, you can plainly see the Ermax decal on the windscreen.
Another good source is Gustafsson Plastics. They made all of the windscreens for the Fast by Ferracci team. Check out:
http://www.bikescreen.com/proddetail.asp?prod=zzEraldoDucati888`92-`93DouhgPlone
The photo is incorrectly labeled. It is an image of Pascal Picotte's 1994 bike (which I own) at the '94 Daytona 200, where he was on the pole.
Brian