Retailers like Net-a-Porter think they have found a way to give online shopping more of the feel of an outing at the mall or an hour with a catalog — by creating apps that resemble magazines for tablet computers. Just as magazine publishers are producing iPad apps that mimic print in a way they never could on ordinary Web sites, retailers are making iPad catalogs, with big, stylized photographs that users can flip through on the couch or in bed. And also like magazine publishers, they are adding rich features like video, sound and 3-D views.