Sorry to resurrect such an old thread, but you may have already figured out they're easily $50 a pop, possibly much more as I've not ever seen one in the original box. I've watched ebay a few times for an extra mag for my '64 MK I and those that pop up are typically around $50 in average/well used condition.
FWIW High Standards does make a nice replacement MK I mag, has a black plastic floor plate/grip piece instead of the original chrome but unlike the current replacement MK II & III mags these are the same width as the original factory MK Is with a zigzag spring instead of the pencil spring. Mine feed & drop just as smoothly as the factory mag.