I'd like to thank you all for your kind and useful answers.
I've contacted Durst directly: I was sceptical, but they have the condenser available yet. By the way, resummerfield is right: its price is higher than the average price of the whole enlarger on Ebay.
I've also checked Northern Photo, I hope they can provide it at a lower price.
Ian, the holder and the other glass are not damaged, so a glass workshop could be a valid option... do you know if the two glasses are identical? In case, I could bring the mint one with me, and ask to replicate it.
Well, these are my first posts on APUG.
I hope I can give my little contribution to this great community... thank you again, and sorry (in advance) for my poor English.
Lorenzo