It says I need Iron Horde Scraps and Crusted Iron Horde Pauldrons to buy this, and that they should be on Iron Horde corpses. But I’ve killed an an absolute ton of Iron Horde Orcs and not a single one has had either of these.
What am I missing or doing wrong?
just line the bolt
The Iron Docks are usually the best place to farm the items.
Just to get the obvious question out of the way, you do have a level 2 War Mill in your Garrison on the character that is trying to get the Iron Horde Scraps, correct?
Edited to remove the level 2 section as the guard transmog can be obtained at level 1.
Also this, make sure you do this.
Ahh, the items were in the shop even though my War Mill was only Level 1 and there was nothing saying I needed to upgrade it. Damn, this is going to waste a lot of time. 
You are correct and I do apologize for my confusion. I was mistakenly looking at the Orc transmog (which requires level 2) instead of the Guard transmog (which can be obtained at level 1).
I will now correct my previous post.