The Diplomatic Pouch is the centerpiece of the Diplomacy hobby in North America. It is the brainchild and love-labor of Manus Hand, human extraordinaire, all-around good guy, and backstabbing pig-dog. Oops. Was that our out-loud voice? From the Pouch, you can find a plethora of articles on Diplomacy covering tactics, strategy, variants, and theory. Additionally, you can find games online, find news about upcoming events, and get in touch with local groups.
Diplomacy World is one of the oldest and most comprehensive sources of information regarding the Diplomacy hobby. There you can find many articles on the game, videos designed to quickly teach new players, information on upcoming tournaments, and more than you ever really wanted to know about the history of the hobby.
Board Game Geeks has a wide array of information about Diplomacy, as well as any other board game you can think of - and quite a few that you really don't want to. They also have an extensive forum with conversations on all gaming-related topics, and will point you toward game stores online and near you.