I found this: https://www.replacementboatparts.com...tesylight.aspx
$13.99, $22.41 shipped to NJ (from FL).
Also looked to see if Lumitec makes something that would cover the hole. Unfortunately there's nothing that is a fit. I recently updated the two lights under the radar arch with their Aurora dome lights, which are white / blue switchable and dimmable. The trim ring operates as a color selector and dimmer switch. Pretty slick little lights, really happy with them.
Edit: looks like Amazon may soon have these, too, called "Optronics 2407765P RV Courtesy Light." Expected in stock 9/25. I ordered the replacementboatparts.com ones since they are in stock there. Will let you know how they turn out, may be a few weeks before I get back to the boat though.