There are several ports of X to the NeXT.  Some are public domain and some
which need to be paid for.

Here's what I know about the versions and their locations.

mouse-X		Public domain version.  This is what I run on my cube.
		Allows use of middle button by pressing both buttons
		simultaneously.  This is the version on cs.orst.edu:
		/pub/next/binaries/X11R4.pkg.tar.
		Also available on cunixf.cc.columbia.edu (128.59.40.130)
		in /Xnext as a compressed tar file broken into pieces.

XNext		Public domain version of X11R3.  Available on 
		sonata.cc.purdue.edu (128.210.15.30):
		/put/next/1.0-release/binaries.

White Pine Software	
		They are working on a pay version.  They have exhibited
		a beta version.

co-Xist		Pay version by Pencom Software, 800-pencom-4.
		Allows switching from NeXTstep to X by moving mouse
		sideways across screen.

price list for the current version of co-Xist:

                                Commercial      Educational
co-Xist                          $249              $149
Motif                            $100              $100
Documentation, X11R4 and Motif    $50               $50
all three above                  $379              $279

There are discounts for buying multiple copies and a site license is available.

A demo version of co-Xist is free and has been submitted for ftp on
sonata.cc.purdue.edu.  Copies can also be obtained from Pencom:
pensoft!co-Xist_support@cs.utexas.edu