Any Quicksilver MX-190 owners here?

If you can post a pic that might help.   Often times mfg and part number is molded on the switch casing.  There are a few places that specialize in obsoletes

https://www.google.com/search?q=obsolete+switches&oq=obsolete%3Bswit&aqs=chrome.2.69i57j0i22i30l4j0i15i22i30j0i390.8926j1j7&client=tablet-android-samsung-nf-rev1&sourceid=chrome-mobile&ie=UTF-8

The original switch is MIA from this amp. The manufacturer and part number is what I hope to get from an MX-190 owner who can take a look under the front panel.

Pretty strange that Mike wouldn't be able to look back at old invoices and see the name of the vendor.

Not really. It was the early to mid 80s.

That sucks, good luck 

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.