Prosthetic tools can be fitted into your shinobi prosthetic arm by the Sculptor in the Dilapidated Temple after finding the required item for each. There are 10 total:
1) Shuriken Wheel -> Loaded Shuriken: found on a corpse in a building just past the Outskirts Wall – Gate Path idol.
2) Robert's Firecrackers -> Shinobi Firecracker: sold for 500 sen by the Crows Nest Memorial Mob vendor found beyond the Outskirts Wall - Gate Path idol.
3) Flame Barrel -> Flame Vent: found in a bonfire in an area to the right of the Estate Path idol in Hirata Estate.
4) Shinobi Axe of the Monkey -> Loaded Axe: found in a small building guarded by two enemies in Hirata Estate.
5) Mist Raven's Feathers -> Mist Raven: found in a pagoda atop a hill accessible from the Bamboo Thicket Slope idol in Hirata Estate. Grapple onto a ledge on the left side of the path, drop into the river, and swim upstream to find a grapple point. Follow the path and break a bamboo wall, then climb to the upper area to find the pagoda. Beware the tough Samurai nearby.
6) Gyobu's Broken Horn -> loaded Spear: found in the Gatehouse, a locked building near the Ashina Reservoir idol in Ashina Castle. The key is dropped by an enemy on a bridge to the left of the Ashina Castle idol.
7) Sabimaru: found in a chest in an area that you must drop down to which is accessed near the Upper Tower - Antechamber idol in Ashina Castle.
8) Iron Fortress -> Loaded Umbrella: sold by Blackhat Badger for 1600 sen. From the Old Grave idol in Ashina Castle, drop down to a building below and enter through a hole in the roof.
9) Large Fan -> Divine Abduction: found in Long-arm Centipede Giraffe's room just after the Gun Fort idol in Sunken Valley.
10) Slender Finger -> Finger Whistle: obtained after defeating the Guardian Ape near the Bodhisattva Valley idol in Sunken Valley.
The achievement will unlock after having the Sculptor fit all 10 items into your shinobi prosthetic.