I just replaced my fridge vent cover today. I ordered it from Amazon, $21.95 and free shipping on orders over $25. I ordered one more small $4 item to get the free shipping. Quite probably you Have a Dometic and the Dometic part number is 65528. I understand Dometic doesn't make them anymore but the Ventmate 65528 is an exact duplicate. Here is the Amazon link:
Ventmate 65528 Polar White Direct Replacement Dometic Refrigerator New Style Vent Lid : Amazon.com : Automotive
It took me longer to get the ladder out and up than it took to replace the vent cover. There are 4 screws holding the cover and they are, no doubt, burried in dicor or putty sealer. It may take some time to dig the old stuff out if it has been there for a long time. Mine was fresh since I just took a wasp nest out of the cover a few weeks ago. One other point, the Sunline screws require a small square driver !! The replacement cover came with 4 new screws that are Phillips head.
Easy job to do.
Here is a picture from Amazon.