This is not a suggestion of what to buy, but for information about finding part numbers/names. Go to this link:
https://www.amazon.com/Dometic-31093.../dp/B004LELY6K
In the center of the page is a section called "Products related to this item" there are a number of photos and descriptions of side vents for refrigerators. As a note, be sure of the brand refrigerator you have. Usually Norcold and Dometic side vents are not interchangeable. So buying the correct brand makes it much easier to just replace the center part, rather than having to replace the entire flange and center vent.