Seattle and Tacoma port leaders are pressing the federal government to follow up on President Barack Obama’s National Export Initiative, hoping to find ways to improve the shortage of containers and shipping capacity that is limiting Washington state exports. In a letter Monday, the port directors cited an estimate by the Agricultural Transportation Coalition that farm exports could rise 20 percent if producers could find containers and ships to carry the farm products. They asked Commerce Secretary Gary Locke, a former Washington state governor, to find a solution.