After matching the photo to a bunch of switch-panel photos online, I think I found it. It appears to be manufactured by (or for) JR Products, an RV-industry supplier. Oddly, I can't find the quad black panel anywhere.
Here's a link to the quad white version on Amazon.
It might be easiest to buy additional or replacement switches from FWC as PaulT notes. I guess the other possibility would be to buy a single, double, or
triple black one off Amazon and move the switch(es) to your existing panel. I assume they're a snap-in fit.
The web site where I first found the JR Products panels and switches is
Trailer Parts Superstore. I couldn't tell which switch applies. Also, I can't tell if any of the the illuminated switches they show would fit the panel (if you happen to be interested in those). Perhaps a call to Trailer Parts Superstore or JR Products (
Link) would clear that up if that appeals to you.
Hmm. And here's an interesting twist. There's not only a
jrproducts.net, there's a
jrproducts.com. And that latter one shows me where I can buy the products and their prices (but still doesn't show a quad black panel). I recognize many of the business names as relatively close to me here in PA. They must be picking up my location from my computer and customizing the list for me.
If anyone has the part-number of the switch and/or the quad black panel, please post.
Also, if anyone has put in a switch or swapped a panel for another, please post the process (I don't have one so can't just go look at it).