For a very informal date in formal gear, go check out some of the costume stores geared toward theatre and television. Better to go to those that also rent to individuals, too, so that they’ll tolerate you trying on all the costumes and laughing uncontrollably. Sommier Brady, in the Passage Brady (20eme), has got a huge selection “from prehistoric times to the future”… Also, if you walk to the Seine from metro St Michel, to your right (without crossing the river) you’ll also find a costume store with beautiful renaissance-style clothes.
