By April Joyner NEW YORK, June 6 (Reuters) - The shares of so-called dollar stores have suffered as U.S. shoppers have started spending more at bargain retailers with focused concepts and big discount chains. Once thought to be insulated from the retail industry's challenges, the dollar stores, which got the name because they try to price items for $1 or less, such as Dollar General Corp , Dollar Tree Inc and Big Lots Inc derive much of their sales from househo