I'm not in the Dark Empire but I know quite a few people that are. They are mostly based in the South and Mid-West region of the U.S. You'll find a couple here and there along the coasts. They are still young but have been growing a lot lately. If you're doing an original Sith outfit, they would be your best resource.
But I can't say enough for the 501st references. Consider some of the materials and patterns available through the 501st. It's a good place to start!