Source patches are available to upgrade X11R6.3 PL2 from the X Consortium (now The Open Group) to XFree86 3.3.3.1. Binaries for many OSs are also available. The distribution is available from:
and the following mirror sites:
Ensure that you are getting XFree86 3.3.3.1 - some of these sites may archive older releases as well. Check the RELNOTES to find which files you need to take from the archive.