Sorvan's site has the best links for the rules from The General for Ix, Tleilaxu and Landsraad, plus lots of great Dune game info and downloads, including leader discs which I used and adapted for my amateur variants.
Rick Heli has created variants for Ix and Tleilaxu, and has links to other variants including a historical 3 player variant which starts with fixed alliances between Harkonnen-Emperor and Atreides-Fremen. He also has a lot of other ideas for variants of lots of games, including the Parker Brothers Dune game from 1984.
Richard Vickery has also created Ixian and Tleilaxu factions, and added some of his own treachery cards.
Flaney has posted accessible player shields in the style of the originals for his favorite rules governing Ix and the Tleilaxu. There is also a player aid pad which accomodates these factions.
Check them out!