What's a unusual but sensible ingredient used to make a Cloak of Protection? Could be something you harvest from a defeated monster or collect from nature or an abstract concept.
If you use the Xanathar's rules on crafting it suggests using a CR 4-8 creature for an uncommon item such as a Cloak of Protection. I tried to come up with some interesting options from that but nothing felt like a great fit (admittedly I only checked the MM). The ok-ish options I came up with would be the hide from a chimera, oni, or some kind of wyrmling or young dragon.
Why not all of these? Maybe the barbarians in the cold mountains of the north use the hide from the native cave chimeras, but the southern elves use powdered copper dragon scales (they trade for them with a copper dragon), and the dwarves use sandworm guts (nasty little side quest)