I've done a search for websites from which to buy a Versaille Day Pass but didn't find anything. Hard to believe it isn't here somewhere...anyway, can it be done? Where?
There is no advantage to buying a pass before you get to Paris. For information on where to buy one, go here: www.chateauversailles.fr/en/index.php.
If you're going to visit other musuems, buy a Paris Museum Pass. Entrance to Versailles is included. See here: www.parismuseumpass.com.
Or go to a RER station and buy a Forfait Loisirs for Versailles. It includes transportation on the Metro and RER to Versailles and back plus entrance to the chateau. See here: http://tinyurl.com/266x9n.