"Impossible to find" as in, the TN is listed as Impossible and won't let you purchase? That could be a purchase level issue. Just because something is on the market doesn't mean you're automatically approved to purchase; selling high end weaponry to every Tom, Richard, and Harry isn't a good way to stay in control of a large interstellar nation, after all. You need to build up your reputation and through that, your purchase rating goes up which allows access to the higher-rated systems.
