I don't know about a Kevin Smith X-Men script... could be.
I heard that Kevin Smith had submitted a script for the upcoming Superman movie. Apparently, he was so insulted with the script the movie studio had dug up (it had Superman brawling with polar bears) that he crafted one of his own, and it was supposedly pretty good. That's just what I heard, it very well could be mostly innacurate or entirely false.
If Kevin Smith was intending to write a script in the hopes that it would be made into a movie, your chances of finding it on the net are pretty much nill, I think. It would be against the interest of whatever company was funding the production of the movie and Kevin Smith as it would reveal all of the twists and turns, thereby spoiling all surprises for anyone interested in seeing the movie. Also, many who were indecisive about seeing it could read the script, be turned off by it and definitely not buy a ticket.
However, if he had written a fanfic type of script just for the hell of it, I suppose that might be one the web.
Kevin Smith wrote the script for superman reborn but had disagreements with producer John Peters (the dumbass wild wild west guy) and was fired by Tim Burton. I don't think he wrote an X-men script but I do know that Affleck says he won't do another Daredevil movie unless kevin writes the script. The Superman Reborn script can be found rather easily on the web.