Where can we buy Navigo weekly pass

Hi Tanya,
 
You can buy a Navigo Weekly pass on Sundays at CDG Airport train stations (either CDG Terminal 2 train station or Terminal 1/3 train station (Roissypole bldg., in between T1 & T3).  Have a look at the linked articles for photos of the ticket office.
 
Keep in mind the Navigo Week pass isn't valid until Monday morning.  You can add a Navigo Jour day pass to your Navigo week pass for use on Sunday from CDG Airport into the city and for the rest of the day.