where can i buy a Navigo easy card in Paris?

You can buy Navigo Easy cards at any Paris Metro station ticket window, Paris RER station ticket window or large train station ticket offices which read: Billets Paris et Ile-de-France.  You can find these large ticket offices at Paris CDG airport or the major train stations in Paris such as Gare du Nord, Gare de Lyon, Gare Montparnasse, Gare St. Lazare, Gare de l'Est.
 
Navigo Easy is not sold from ticket vending machines (yet, as of 2019).