I have planed to take taxi from airport to Disney and stay couple nights there. (Sun - Tue)
I will be leaving Disney Area to go to Paris on Tues and want to use weekly Navigo Decouvert.
I am not sure if I can purchase the card in that area? Which station will be better?
You can buy a Navigo Découverte card and put a week pass on it at Marne la Vallée RER train station at Disneyland Paris. You would do this at the RER train station ticket window.
There is a photo of the Disneyland Paris RER train ticket window in this article on Navigo Découverte (see "Where to Buy Navigo" section).
I'd recommend you buy your Navigo Découverte at CDG Airport train stations when you arrive though, since there are multiple ticket windows & offices which will have the Navigo cards for sale. There is a small chance that the Disneyland Paris train station could run out of blank/new Navigo cards and you will have no other option than to try again in Paris after using a regular RER ticket to get into central Paris.
You can buy the Navigo card with a week pass loaded onto it at CDG Airport upon arrival.
By the way, you may want to consider buying a TGV train ticket from CDG to Disneyland Paris as it would be significantly cheaper and the travel will likely be quicker as well. The section entitled: Buy CDG - Disney TGV tickets online on this page.