The converter dolly in the photo was fabricated by the gentleman who is unhitching the flatbed from his Mog... Several folks on this site have fabricated dolly's, some using M105 trailer chasis (like this one), some using heavier generator trailer chasis...
They show up now and again at GL sales. There are both single and dual wheel models in the inventory.
Commercial outfits also use them when pulling doubles and they are up for sale often. I find listings for them often in various equipement trader publications and in Rock and Dirt. Only problem for some folks is that most of the commercial units have 22.5" rims or similar (so, you'd have to do some swapping out to get them to match your MV...).
The converter in the photo above left my yard the day after this photo was taken, the flatbed is still in the yard waiting an assignment hauling a XM708 around behind a M817.