Rocketry.Org maintains a large web database of suppliers for anything experimental rocketry enthusiasts could want, as well as a global rocketry organizations database and book lists.
The Chronicles of Experimental Rocketry. Contains several personal rocketry projects that go into detailed explanation that anyone starting in scratch-built rocketry might be interested in.
SPL develops propulsion elements for sounding rockets like LOX valves, injectors, regeneratively cooled combustions chambers, igniters, propellant feeding systems and thrust test stands.
Tripoli is a non-profit organization dedicated to the advancement and operation of non-professional high power rocketry. Tripoli has members in the United States and 20 countries around the world.