I share your pain
I too have owned a few of the old MON Series antennas. They blow away anything I've ever owned from a wide-band perspective. The famed Austin Spectra, which close, is not as good on low band. The MON antenna with that metal tube in the middle (A/S called it an "800MHz Micro-Choke" or something like that) really helped pull in those trunking signals. There ARE better low band, high band and 800MHz trunking antennas - but none out there right now does it all like the old MON Series.
I have also been scouring the web for ANY of these. I saw one last month on ebay, got off the site, thought about for a few days, went back and it was sold.
I think you're only shot is to just scour places like ebay and/or craigslist. That's what I'm going to keep doing....good luck!
-John