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Weekly Heroic, Vanguard Tiger, Nightfall, Gold chest on mars, Public Events
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Nightfall and weekly heroic strikes can get you 18, public events and pvp rewards can give you them too. The last way is from rare engrams- but is not very common. [spoiler]Tip: use three characters and transfer armour via the vault so you can level your hunters past level 20 quickly.[/spoiler]
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Nightfall. You can also get them from rare engrams and as daily PE rewards.