You gain Lightbringer points in the same manner you would gain Sunspear. After a certain point in the game, you no longer see Sunspear Scouts offering bounties in explorable areas, and instead see Whispers Informants, who will give LB bounties. There will be some areas where there are LB and SS in the same zone, but most end-game zones, specifically the Realm of Torment, will be LB only. Grab bounties and kill stuff.
Or, instead of bounties, you can complete missions, do Zaishen Bounties that reward LB points, do other quests rewarding LB, or turn in completed story books.