Hyperion and (maybe) Monitor are very rare ships. Hyperion can only be found in Tibicena. Monitor can be found at more military markets (at least Ludd and Hegemony), but it is otherwise about as rare as Hyperion. Tempest is also another rare ship, only a bit more common than Hyperion.
In a game with a level 74 character, I found only two Hyperion for sale in the entire game (and two more in enemy fleets, where I save-scummed until they were captured), and the first one that appeared was when my character level was in the 50s.