That's a very good question. I don't think you can purchase a Mobilis pass at a Tabac, but I'm not really sure. I've also never seen them available online. Just in case, I checked Rail Europe, and they don't have them. They just have the Paris Visite, and to my knowledge, that's the only pass available online: http://raileurope.com/us/index.htm

According to the Europe for Visitors website, Mobilis passes are available at major Metro, RER, and train stations, so I would guess the answer is no. It would certainly be convenient if they were available there. Maybe someone going to Paris soon can check. http://tinyurl.com/nsex3

Best, Sandy in Illinois