Mora is a pen shop, not a stationary shop. Although it is worth visiting, of course.
Skripta -68 Rue du Cardinal Lemoine - 75015
Papeterie Saint Sabin - 16, Rue Saint Sabin - 75011
Styl'Honoré (Mr Arabian is a very nice person) - 1, Rue du Marché Saint-Honoré - 75001
I'd like to echo the recommendation for Papeterie Saint Sabin. It's in my neighborhood, and carries Lamy, Pelikan, Waterman, Caran d"Ache, Yard-o-Led, Graf von.., Aurora, and a few other brands of FPs. There's a very decent collection of stationery, folders, inks, and other writing stuff. They'll give you a 20% discount across the board, and more if you talk to them nicely. BTW, as my wife would testify, they also carry a very nice selection of handbags etc (Mignon, Groom and other hard to find but quintessentially French brands). I've had nothing but good experiences with them.
Mora Stylos gets rave reviews in this forum for what they carry and for their service. Unfortunately, I've never had good experiences with them. On the three occasions in the last 7 years that I've dealt with him, I've found M. Mora to be cold, impersonal - and on the last occasion - downright hostile. I guess I'm not their type of customer.
There are many other very nice stores around the Opera area. Plume et Bille (rue de l'Arcade, I think) and Opera Stylos (don't remember the name of the street) are worth a visit.