Here are some sites you're sure to like: ErectorSet.Net
The A C Gilbert Heritage Society Website: The ACGHS is THE organization for Erector.
Wagner and Sons: Highly recommended if you need original parts.
The Smithsonian Magazine: Bruce Watson's excellent article on A C Gilbert and Erector Sets.
Paul Linford's Erector Set Website: Pictures of Paul's collection and other good stuff.
For reproduction parts, email Joe Long at wwiz@juno.com.