I would start with the water heater manufacturer or the RV manufacturer. RV supply stores might be able to help, but they can't stock every cover for every appliance in every rig. Amazon and eBay are also good sources since they have hundreds or thousands of RV related vendors.
There's a RV surplus parts place in Corona, California called Walts RV on Railroad St. It's somewhat organized, but always a bit of an Easter egg hunt to find exactly what you're looking for. Don't worry about the color, you can always spray paint it. Walt's says they will ship parts. Call them with exact dimensions and see if you get lucky.
As a side note, Walmart sells "fake vents." It's a rediculously silly idea, but if a tree or something damages your RV, and if the damage is not too big, you can cover it up with their fake vents. I suppose it could work if it looks okay and you don't abuse the idea and end up with random vent panels all over your rig.