You can't get the genuine parts from Ducati now. You've got a few options:
1. stay with the Strada seat frame, taking lots of care with the bits that stick out, and use one of the aftermarket GRP mouldings. That route has been used sucessfully by a number of the 888 owners on the board and is the cheapest. I'm sure one or two will be along shortly to advise on the better/best suppliers.
2. eBay. Wait and watch. The genuine ally subframes and seat units come up from time to time, but be prepaerd to pay large. NB. The 851 ally seat frame is much more common and will not do the job
3. I have a new, genuine Ducati SP0 steel single seat frame in the loft. It owes me quite a lot but it's spare.