The Model M certainly is a cool vintage keyboard, though I was hoping to find something that can still be purchased new, and in the generic smaller footprint of the 'Filco' style.
Though I am guessing that may be asking for the impossible.
Rosewill does include a Mini-USB to PS/2 cable with their RK-9000 keyboards, but they do not seem to sell it separately.
Show Image
(http://i89.photobucket.com/albums/k221/undarken/Rosewill/Cables.jpg)
Perhaps you could get someone like Pexon (https://geekhack.org/index.php?board=146.0) to make one for you.