The Madu's Shield is only used by Aya during naval missions.
The Medjay Shield can only be bought from the Blacksmith in Siwa if you do not have the shield on your inventory.
The clean version of the Medjay Shield is only available in the arenas.
A staff, Bringer of Chaos, and a predator bow, Valkyrie Operator, are completelyunobtainable despite being known about.
I was also interested in getting this shield, it's really beautiful, it's a shame it can only be used by Aya. Or she should have had a bigger role in the game, as originally planned - few know that Bayek was supposed to die about halfway through the story. His wife was supposed to revenge him and continue as the main character. It would have been something like AC3. However, Ubisoft scrapped that plan.
Btw if any of you want to know about the best shields in Origins, check my article out:
[Top 5] AC Origins Best Shield And How To Get Them