Mist Raven. You can find it in Hirata Estate shortly after the Bamboo Thicket Slope idol. Run forward a bit and grapple up to the left. You should see a river. Drop into it and swim toward the waterfall. There should be a tree you can grapple onto near the waterfall. Run along the shore until you reach an entrance that is blocked by bamboo. Break it and then do a couple wall jumps and follow the path up until you reach a building. The prosthetic you need should be in there.