Ducati buy their belts from whoever wins the contract.
The key is not using old rubber, so buy from a reputable source who has some turnover.
I get most of mine from john fallon at mdina italia
www.mdinaitalia.co.uk cos he sells them cheaper than ducati ones.
If you look at some of the reputable american ducatisti-sites, there are lots of guys using non-duc belts. eg californai cycleworks
www.ca-cycleworks.com I run belt drive on my classic racers, and have had 1/3 of the belt wear down during a race meeting without me noticing (due to stone pickup !). Its how good the rubber is, not who you buy it from !