Where in Paris can you purchase "Le Passeport" tickets to Versailles?
You have two choices for passes:
(1) "le Passeport" which covers admission only. For points of sale, go here: http://tinyurl.com/2usdvn.
(2) a "Forfait loisirs" which covers tranportation on the Metro & RER to Versailles and back as well as admission. For more information, go here: http://tinyurl.com/y84c3o. I don't know if all RER stations have Transilien offices, but the major ones do as do all the mainline train stations.
A third option is to buy a 2-, 4-, or 6-day Paris Museum pass. Entrance to Versailles is covered, but not audioguides or the weekend fountain show. See www.parismuseumpass.com.