They are, indeed, rare, as they're part of a set of Random World Drops that used to drop on Voss, Corellia, and Ilum. With the changes to loot drops at the release of Game Update 4.0, however, gauging what drops where from who is mostly just a crapshoot.
That said, it's not a terribly popular set, so it should be cheap once you find one on the GTN.