We as a community have been suffering for Epic Ingots ever since Weapon upgrades were added. Since PvP only provides Ingots based proportionally on rarity, and Delve rewards only Legendary and Mythic ingots, I have long since finished all the Common to Ultra-Rare weapons, have no trouble buying Legendary affixes I want, completed every Mythic a long time ago, and am nowhere near close to upgrading all my Epic weapons, despite being more than a year later at this point.
The devs have stated the Adventure Board will, going forward, be a method for the team to trickle resources in to players. They even explicitly stated the Adventure Board would have them, but we know the list of tasks and it doesn’t.
The new Challenges grant Ingots at Tier 6, but this is only enough for… 146 Epic Ingots, or a little more than 4 weapons total. Besides that, my only sources of Epics are a couple from weekly Tier rewards and a couple from upgrading lower level ingots, which means only a couple per week, of something we need hundreds of.
Hopefully we see more sources of Epic Ingots in 4.6.