Just a map in high resolution that I made for myself, maybe it can help. It has all the royal weapons locations besides the ones you all ready get from the main quests, the last one been acquired after Chapter 13.
Don’t mind the numbers, they suggest my personal organisation for doing this all in one run with no sleep and doing quest on the way. But if you would do this, i acttually advise that you start with the one by The Vesperpool and Myrlwood. Because there’s a quest for the Regalia that makes it faster, so it can help doing the others. Also the one in Malman Thicxet has no number because I have done it along the main history in a side quest quest when you are taking Iris to Caeum and the group sugest to stop there to explore.
Getting all gives you a Steam Achievement and a special Royal Arms Ability as well (since one patch that idk wich)
Hope this help someone. Good gaming!