First you save your game and then go to the nintendo menu and change the date to a different season. Go to the exhibhition hall and buy whatever you like. Then go to your apartment and save. Change the date back to the real date and go on style boutique again. It will say something like change the date back to the time you last saved to keep any events. There will be two options, continue anyway or change the date. Go on coninue anyway and go to your wardrobe and the clothes will be there! If you changed the date to a sunday and bought out of season items at a preview event, you will have to go on the next day, accept the delivery at your shop and save.