Onyx is the brand of armor that Shepard begins with, because it is a lower grade armor it will drop through the entire game at an uncommon basis. To get the "X" grade armor, you'll simply need to be a very high level for it to appear inside item crates or off a defeated enemies uncommonly. You can also try checking armor merchants when your character has reached a high enough level. Revision X items start appearing to the player at level 55.