I know, it sucks. those parts are cheap pot metal, NOT something like stainless or aluminum. they break very easy. I've always wondered why Ducati put such cheap bits on their bikes. Price, I guess.
I asked 2 different dealerships (one which had years and years of Duc experience) and they both said they couldn't get those little covers. If someone is parting out their bike, you should be able to pick them up. Should be the same for all Monsters 620, 800, and 695 (all the early fuel injected).
2003 Monster 620ie (The Silver Surfer)
Stuff bolted on of questionable value