Any Quicksilver MX-190 owners here?

You can try Mouser or Digi - Key and try to match it.

Yes, try Mouser.  These are not exactly the same, but getting closer. Here is a filter set for some of the Carlington Technologies Rocker switches as a starting point. Looks close to the stock switch, to get some ideas. Just go with a quality switch, many of the parts are generic on those older amps.  You could call Mouser and ask for a tech. Toll Free:(800) 346-6873

 Example:  

Thanks. Yep, I've searched Digikey and Mouser. And Newark, Arrow, etc. The specific OFF/ON/ON configuration I need is not in the usual parts places. My best chance is going to be to find out who the original manufacturer was and try to track it down used or from a surplus place.

I'd just measure the cutout and find one that works. It really doesn't need to be identical looking. Knowing Quicksilver, it's just a generic switch. 

You're right that it doesn't need to be identical, but it does need to be OFF/ON/ON, which is extremely uncommon in any switch, let alone a rocker.