Hi Steve,
Navigo Découverte pass cards are available at CDG RER train station ticket counters. If you're very unlucky, they might run out of the plastic radio frequency Navigo cards, thus you'd have to buy one when you get into Paris and simply use a regular
Billet Ile-de-France to get from
Charles de Gaulle Airport to Paris.
Do check out the
photo tour of CDG Terminal 2 to Paris and make sure you go to the correct ticket counters for buying your Navigo Découverte pass.
Some friends recently had a ticket seller hassle them about the
Navigo Découverte being for France residents only and that visitors should buy the
Carte Paris Visite. This is nonsense.
If you want to prove to the ticket seller than you have a right to purchase the Navigo Découverte, download, print, and bring with you the
Guide Navigo from STIF (the parent organization of all public transport services in the Ile-de-France region).
I've translated the parts which are interesting in the Guide Navigo on the
Navigo Découverte Carte Orange page.
There is an automated photo booth at the CDG Terminal 2 train station. 4€ for 4 photos. The ticket sellers often have scissors handy for trimming photos. If you can bring your own photo (of whatever quality, it really doesn't matter), that avoids any hassles with a possibly broken photobooth or having euro coins handy.
Best regards,
Ben