Legend and master lost sectors are on loot rotation - each day for one of the 4 armor slots to then repeat. Even if not displayed ingame, you should still get exotics, though beware, the droprate is quite low (estimated to be ~20% on legend and 35% on master - platin (all champs defeated) that is). You can also find the rotation info here: https://www.light.gg/ Simply click on 'details' next to the sector symbol. The exotics shown are not the whole of what can drop but you get to know what slot they are from today. In order to obtain them, you will need the expansion of the year where the item was released in - in this case Witch queen. Other than that, you can do Vex strike force on Neomuna, which is (to my knowledge) guaranteed to drop an exotic that you haven't obtained yet.