Fenders are easy to find, but I wonder if Nissan even makes quarter panels for the 240SX anymore. I doubt it.
The best bet would probably be to take a quarter panel from another 240SX, if you really need another quarter panel. By my experience, there is NO quarter panel that is unfixable. Furthermore, repairing a badly damaged quarter is almost always easier than replacing it. Welding a quarter panel absolutely sucks, because the alignment is really hard to get right. You only get one shot.
Why do you need quarter panels? I'm a body man, maybe I can steer you in the repair direction.