Nails is an item in The Witcher 3: Wild Hunt that comes in two different forms:
Nails (crafting component)
The crafting component is needed to craft the following items:
It can be found as random loot or purchased from the following merchants:
- All armorers and blacksmiths
- Armors and weapons merchants in Nilfgaardian Army Group 'Center' Camp
- Merchant in the boatyard south of Refugees' Camp
- Merchant at Calveit Watchtower
- Merchant at Coast of Wrecks
- Merchant in Ursten
- Merchant by the road west of White Eagle Fort
- Merchant at the sawmill northeast of Rannvaig
- Merchant at the Isolated Hut on Hindarsfjall
- Merchant in Mohrin Village
- Merchant in Stacheier
Nails can also be obtained by dismantling:
Nails (quest item)
The quest item is requested by Octo for the ship he is buiding during The Lord of Undvik.
Some can be found near the boat hull itself, some on the other side of the barricade, west of the shipyard rail and about halfway to the water, and then more are around Dorve Ruins. They come in large stacks and, if picked before the quest completes, later turn into junk that can be sold for a total upwards of 120 
.