The Loaded Umbrella is one of the more versatile and useful Prosthetic Tools in Sekiro: Shadows Die Twice. Finding it can be a bit hard, though.
If you don’t have the ‘Old Grave’ Sculptor Idol, you’re best bet is to start at the ‘Upper Tower – Antechamber’ idol. From there, head out the window you used to access this location and work your way backwards. You’ll want to get on top of the roof directly above the Antechamber Sculptor Idol. Once there, look out across the rooftops away from the Sculptor Idol. You’ll see two high rooftops, with a rooftop a bit lower than those, connecting the two buildings. That’s your goal for right now.
Make your way over there in whatever fashion you see fit. It’s not very difficult or painful to get over there, and you shouldn’t have many obstacles or enemies in your way. Once you’ve made it over, look straight down and you’ll see a Sculptor Idol and just beyond that, slightly to the left and another level down, a hole in the roof of a building.
Make your way down to that Sculptor Idol, and activate it. If you’ve already gotten the ‘Old Grave’ Sculptor Idol, simply start your journey here. From that idol, jump down into the building through the roof and speak with the merchant in there. He’ll be selling the Loaded Umbrella Prosthetic Tool for you. Cough up the required Sen, 1,600, for the Iron Fortress, which turns into the Loaded Umbrella for you!
From this point, simply head back to the Dilapidated Temple and have The Sculptor fit your new Prosthetic Tool onto your arm. Just like that, and you’ve learned how to find and equip the Loaded Umbrella in Sekiro: Shadows Die Twice.