Did you try booting from the discs and doing a custom install?
X11 is not part of the regular install of OS X.
You need to select custom install and modify the install options just to install X11 only.
"X11 for Mac OS X provides a complete X11 Window System as an optional installation
Look on your installation disc. Use Pacifist to selectively load program from disc.
Restore Tiger 10.4 & Leopard 10.5 DVDs are available from Apple by calling 800-767-2775 as of January 20, 2013.
did you try fink or MacPorts?