Flash sale sites, only a few years old, are tremendously popular. American visits to them increased 68 percent in September compared with the same period a year ago, according to Experian Hitwise, an Internet tracking firm, and 314 percent from two years ago. Such sites enable customers, who are about to enter the busiest shopping period of the year, to score deals on elite brands. Still, one can, with surprising regularity, find some even better deals elsewhere.