The main website.  If you're interested, you can purchase miniatures and the Ironclaw/Jadeclaw books from the site.  They also have various summary sheets, sample characters, fonts, sample characters, character creation guides, errata, a few paper miniatures, and other goodies are available from their download page.

Jordan "Greywolf" Peacock has provided lots of world material for Ironclaw.  It may not be official, but I'd hazard that the star atlas and zodiac would definately add flavor to your adventure.  He's also attempted to catalog the many minor houses that are scattered throughout the Ironclaw books.  In addition to the game material he's created, Mr. Greywolf has hundreds of paper miniatures available for download!

If you use his material or miniatures, be sure to drop him a line telling about it!

Yahoo hosts the Ironclaw mailing list.  Lots of interesting information gets thrown around here.  You can read the messages without needing to create a Yahoo account.  If you do create a Yahoo account and join the list, you can use the database to see if any other Ironclaw players are in your area, or take a look at various uploaded files.  Sometimes, the good folks at Sanguine will post material for beta-testing here.

The official World Tree web page, they explain what the game is all about.  They have links to in-depth reviews.  Errata, character sheets, sample characters, and a sample scenerio are also provided.