Of course, the obvious first step to take would be a simple phone call to VPI to check. As Stanwal says though, it's doubtful they do.
But all is not lost, this Black Acrylic material can be gotten, whether you are using a standard 1/2" thick Armboard, or what I figure you are more than likely using with the MK-IV, is the Super Armboard, which is stated to be 1.00" thik, but might be actually slightly less thick. Take a measurement to be sure.
This custom Armboard work could then be done by a competent machinist, first making a clone-copy of size, drilling Armboard Mounting Holes, mount-attach Armboard with Screws, and then do a precise Spindle-Pivot measurement for whatever Arm you desire Then drill the required size hole-holes for Arm-Arm Base, and you're done.
Be sure to remember to ask VPI if they cannot do the machining-drilling for particular Arm, do they at least have the blank Armboard, so you can have the finishing work done your end? Hope this helps. Mark