You will find Fnac and Virgin Megastore box offices throughout Paris, where you can buy tickets for shows and for temporary axhibitions in museums, that will cut down on queues before entering museums.
Good to know : The Kiosque Théatre sells half price tickets for shows playiing the same day you buy them. There are 2 spots, one on the Place de la Madeleine, the other one in front ot the Montparnasse train station