Races of Eberron is where you'll find the armor-less warforged feat. It works kind of like the Mithril or Adamantine armor feats...replaces your composite plating feature with some other stuff, including, in this case, spellcasting free of arcane fail chance.
Spellsword...I am not sure! Probably Complete Warrior or Complete Arcane. Don't have books here with me now.
-- Edit: The Hivemind informs me that it's Complete Warrior.