I doubt it is available as a seperate part. I'd call Alfa Parts Exchange
to see if they have a dead AFM they'd be willing to yank the cover off for you.
The original would have been glued on with some fairly strong adhesive. My guess is the cover had been removed to 'adjust' the AFM's flap door spring. Then whatever glue was used to replace the cover let go.
No reason not to fabricate a replacement yourself if that is within your abilities. It is really just a dust cover. Are you gonna make a chrome plated cover to add some bling?