No, you can't download X11 for 10.3 from Apple -- you'll need the original installation CDs.
I would recommend contacting the seller of the Mac and inquiring about getting the disks -- since the license is basically tied to the Mac, there's no point in not giving you the CDs along with the computer -- unless, of course, he/she lost them, which is a different problem altogether.
A Mac without installation CDs is like a car without keys -- I would highly recommend getting your hands on a legal copy of OS X for your new Mac.