Camping World has them. But you will have to ask for a better one than what is on the shelf. The ones on the shelf are flemsey. You will need some clear silicone to put behind it if you are going to replace it yourself.
If you don't lock the tabs on the vent before you leave there's a good chance you will loose the flapper inside the vent, at least that's what happen to me.😊 I had to replace mine.