Captain Crimson’s Trimmings is only available by crafting at the Blacksmith.
You never know what kind of mood he’ll be in:
As far as the Cube goes, you could also Reforge a Crimson’s item for a chance to get an Ancient or a Primal, but it’s “cheaper” to just craft them until you get a decent one. You just can’t upgrade Level 70 Rare items to get crafted Legendary items.