The best place to buy French magazines is at the Hudson News in the Grand Concourse of Grand Central.They have a wide variety of magazines from news (Express, Le Point, Le Monde, le Figaro,...) to women/men magazines (Figaro Madame, Atmosphere, Elle, Biba,...) to cooking or Decoration magazines.
Otherwise, all over the city, Universal News is also selling some magazines from France. The one I recommand is on Broadway and Broome where you can sip some coffee while reading your magazines.
Don't forget you can always subscribe to most of the magazines from France. They might arrive a few days after normal delivery day in Paris.
And if you just want to look at your magazines without buying them, go to the Library at the French Institute Alliance Francaise...