As hemiken said, make sure you have exactly the right bolt pattern, which is 114.3 on 5. However, another concern with using later model rims is the offset. The Nitro may have 16x6 wheels, but if all of that 6" is offset inboard, then they will hit the springs, control arm, etc. Plus they will tuck the tires in further on the body and not necessarily improve the looks of things.
There are number of vehicles using that pattern, so with careful searching, you should be able to score something on craigslist. Start with a search on the bolt pattern to see what else you should pursue. Some Jeeps are one that comes to mind, some Fords from the 80s and 90s,some Mistsubishis. look around and verify. That will really open up your options.