United Natural Foods, a SpartanNash competitor, is exploring a bid to buy the entire company, the sources said. Oak Street, a Chicago-based investment firm, contacted SpartanNash last month to reiterate its interest in buying real estate from the company worth up to $1 billion, shortly after two activist investors began pushing SpartanNash to explore changes, including putting itself up for sale, the sources added. There is no certainty of a deal, said the sources, who asked not to be identified in order to discuss the private conversations.