WindowMaker (as suggested above) is just a window manager. Whilst it can look quite a lot like NextStep it does not provide the underlying NS Objective C libraries (what is now Cocoa on OSX). GnuStep provides an open source clone of OpenStep (what NextStep was renamed as). It provided an almost complete implementation of the FoundationKit and AppKit, a NS-like window manager, a NS-like "dock", the works.
If you want/need real NextStep, try eBay and the like.
I don't think that there was ever a version of Next that ran on Apple hardware, just the original "black" version and the latter (once it became OpenStep iirc) for Intel. You might be able to get the Black version to run on Apple hardware but I doubt it. You could pick up a dirt cheap Intel box (remember that OpenStep was around a long time ago) and run it on that. Or install fink on your apple and install GnuStep on there...
If you decide to get OpenStep for Intel, try to run it on a computer of similar vintage. I doubt OpenStep would run on modern x86 machines with their AGP and USB etc. technologies that weren't around when OpenStep was.
I don't believe there was ever a PowerPC version of Next/Openstep, so that rules out running it on a Mac. If you want to be really obscure, you could get an old SPARC workstation.