Exhaust Vent Cover

[Wade Sikkink]

Our building is older and has 3" ducting for the bathroom exhaust vents. Two of them that exit the building from the ground floor are missing the hood/flapper that keeps birds, etc. out of the duct. 4" stuff is easy to find, but I can't seem to find one for a 3" duct. I found one that was a 3" duct, but if it's exactly the same size as the duct it won't go in. It needs to be slightly smaller.

Any tips on where I could find such a thing?