Now that raids are world bosses are available, you should know how to get your hands on those nice extra rolls. In Legion, you can use the item called Seal of Broken Fate in order to get one extra roll and a chance at the extra loot. You can only obtain 3 of these per week, and the cap is at 6, so you can save up some for the next week if you don't have time to spend them the same week.
In order to get these seals, you need to talk to Archmage Lan'dalock in Dalaran. He is at 58,67 in the entrance to Violet Hold. He lets you purchase these extra seals for either gold or Order Resources. You can trade 1,000g and then 2,000 gold for a total of 3 seals, or 1,000 Order Hall Resources and then 2,000 Order Hall Resources to get them.
The quests reset ever week, so make sure you pick up your new seals before you start killing the world bosses.