I got my rear sprockets (38t and 40t) with carrier from
http://www.ducati-shed.com/ for a total of £118.50
I bought a new front 14t from Ducati (£20 I think) and then a Tsubaki Sigma-R chain for £70 from B&C Express -
http://www.bandcexpress.co.uk/ You'll need a 96 link chain if you change from the standard 14/38 setup.
How come you've got a 15t front then, and what are you running on the rear? The only 748 that runs a 15t front is the R I thought...