The Malachite Shards drop from any treasure chest.... My girlfriend sent me a cold harbour 1 she acquired at level 46 I'm vet 16 and it dropped me a shard.
You need ten to make one ingot so to speak and are bound once you refine.
At the moment they are inflated due to demand but can find them between 5-15k each.
Me and my Guild on Xbox One have just unlocked Glass Belts and are already stocking up in the Shards.
If you're on the same console and maybe want to try our Guild let me know.
You can't search for them directly on the guild traders. The best way is to set the item quality to epic and set the minimum price to about 5k and work your way up the list. The last time I checked the cheapest was 8k on PS4 EU.
You get them quite frequently from the random loot chests in IC and from the chests after clearing the portals etc.