Not sure if this is common knowledge, or not.
So I’m not really an altoholic or anything, I barely know how to get sparks for crafted pieces, flightstones, or higher level crests.
Apparently, each boss in the mega dungeon is guaranteed to drop a splintered spark fragment if you’re behind. And that goes for heroic mode, too, not just mythic. I spent many hours over the weekend farming low keys, LFR, and various weekly quests trying to get sparks for crafting, but all you need to do is queue for the heroic mega dungeon.
And you can just keep queuing, too. The dungeon doesn’t care if you ran it 15 minutes ago or 24 hours ago. It’ll just keep spitting out spark splinters and flightstones (like 260 per wing?) until you’re caught up. This is amazing for alts. It stopped giving me spark splinters when I completed my 10th spark, so maybe that’s the cap.
Would still like to be able to trade my leftover crests to alts, though.