I doubt you'll find the original fork, however there is this:
http://www.universalcycles.com/shopp...&category=1640

I have this same fork (bought for another project), as well as an '85 Voyageur S/P (ugly blue). I just checked and it's pretty close in measurement to the stock fork on my S/P. It only has single eyelets below the fork, but IME most lowrider racks work best mounted on the lower eyelets anyways.