Empires in Arms was originally published by the ADG (Australian Design Group); it's second edition was re-edited for the Avalon Hill Game Company (today hold by Hasbro, and almost defunct).
The original EiA's Webring at Bomis.com, maintained by Ed Glamowski (contains news on the EiA computer game)
A bunch of useful info can be found at Web-Grognards (The site for wargames on the web)
Christopher Tate maintained an Unofficial EiA Page [Link broken!] with forms, official game errata, game variants (including the Americas, the Advanced Naval Combat, Britain's overseas fleet, Hidden movement and the USAF Academy Rules)
CB's EiA Page. (A rules index, a 1806 scenario, a leaders list (1805 & 1792).)
The EiA Bastion has collected almost all the stuff for EiA available on the web!!!
Game Variants
Mike Treasure's Empires in Harm is a sort of Unofficial "Advanced Empires in Arms". The new rules (v4.0a on 10th of October) are available, along with a discussion group at Yahoo!Groups.