Before traveling to visit his parents in Nebraska last winter, Jeremy Duensing consulted what he always checks before a trip: the "McRib Locator" website.

To his delight, he found a McDonald's restaurant near Omaha that, unlike most of the burger chain's 14,000 U.S. restaurants, had the McRib on its menu. He bought six of the pork sandwiches, ate one right away at the restaurant, and carried the rest home to Burnsville, Minn., in an ice-packed cooler.

"Either...