3oC engrams, raid drops and zur. Only possible ways. Old raids can drop old exotics too. In vog, the gorgans chest and atheon have a chance to drop exotics. In crotas end it's deathsingers and Crota. You can solo crotas end in about 20 minutes with some practice. But it's still rng. In the same time you could do 2 heroic strikes or crucible matches with 3oC active. Of course with engrams you could end up getting armor but old raids only drop guns.