To start your search put a wanted ad in the swap meet section here, (not the general forum)
If you're trying to figure out exactly where to find one then good luck. I don't know of any one make or model of truck that always had one, they were options and you just have to search junk yards and see where and in what they turn up.
Spicer also made larger auxiliary transmissions which may or may not be better for your application, how much HP and weight are you dealing with?
I have owned a 5831 that I bought cheap and sold for I think $250, it needed a rebuild and had no "extras". finding one with factory mounting cross members, shifter, and some drive shafts to work with can cost more and be well worth the money compared to fabrication and scrounging that stuff later.
Keep asking around and keep your eyes open, you ought to eventually find what you want for more like a few hundred $ and not a few thousand.
