Try GnuStep
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.