Duke Nukem 3D includes an installer that will install the game from either the PC "Atomic Edition" disc, or the MacSoft disc from the Mac Classic version of the game. If you don't have a retail disc, the installer can give you the free shareware episode.
Duke Nukem 3D is a fantastic 3D shooter game.
3DRealms still sells the game off their website at 3drealms.com (in fact, they recently did an updated print run, believe it or not). If you don't have it, this is the path of least resistance to a legitimate copy of the game. eBay is another option. Make sure you get the "Atomic Edition" if you buy a PC disc.