I have a couple of the metal ones robbin posted. One for dryer vent and the second for small exhaust fan for our greenhouse.
They are much better then the plastic vents. The flapper is tilted and weighted. However the build quality is mediocre but they do work better then the normal plastic ones. Nothing is going to seal totally.
The other nice thing is they protrude an inch or so warn air does not tend to hit the wood below the vent as with ordinary ones.