According to the FAQ on slackware:
How do I add X to this?
Assuming you've got the space to install it, download the X packages (the files ending in .tgz) from this directory:
ftp://ftp.slackware.com/pub/slackwar...e/slakware/x1/
You might also grab the diskx1 file, which describes each of the packages. You can save disk space if you don't install all of the X servers (you only need the one for your video card), and you might also leave out extra fonts, old X shared libraries (oldlibs*.tgz), and possibly development tools if you're not planning to compile X software yourself. Once you've collected the packages you plan to install into a directory, run this to install them:
installpkg *.tgz
Before running X, you'll need to set it up with 'xf86config'.
Found at slackware.org