Where was Sword Art Online published online?
You will not be able to (legally) find the web novel version anymore as the author has pulled it out after agreeing to publish it with ASCII Media Works. As for the website it was published in, it would be on SAO's author Reki Kawahara's website.

Keep in mind that he used the pseudonym "Fumio Kunori" when he was writing this particular web novel series. 

Source:

http://en.wikipedia.org/wiki/Sword_Art_Online#Light_novels