Empire, and buildable only on Kuat. As with many planets in the mod, you first have to build the shipwright - in this case, Kuat Drive Yards - before you can build special units such as the Eclipse and Sovereign SSDs.
And by the way, that's a HUGE strategy point for the Rebellion. Take Kuat early, and you won't have to face giant, mobile coaxial lasers blowing up your space stations and MC90s in one shot. Each faction has such issues, though - with playing the Empire, it's "take Sullust (and prevent the construction of SoroSuub) early and you won't have to face a pack of Liberator-class carriers and Quasar Fire Cruisers sending swarms of fighters and bombers at you from deep inside the fog of war". As you know, such swarms are the biggest threat to those SSDs you desire.