just keep plugging away at whatever the hardest content you can do is, exotics seem to come from anywhere, so leave no cache unopened, leave no elite mob unkilled, and eventually you'll get it.
best of luck that your exotic engrams dont turn out to be an Aeon cult arm piece. i dont know what bungie was thinking when they designed those, or what they were thinking when they said "Let's make sure everyone gets at least 1 Aeon piece and tune it's drop rate maddeningly high so everyone will have 5 of them."