You can find the "Special Iron Ingots" needed for crafting the weapons that are in the "Special" list of the crafting menu, such as the "Broken Sword" Knife and Washington's Battle Sword (Replica), by doing Homestead missions and by looting chests throughout the game. However there are not enough to craft all of the weapons even after getting all the chests and doing all the missions. You can find more ingots behind the manor at the Davenport Homestead. When looking at the front door of the house, go around the right side. Keep going straight back all the way to the cliff. Go directly behind the horse stable to the edge of the cliff (there are two stables; select the one closer to the house that sits near the cliff). Drop down to hang from the edge of the cliff and look down. There should be a small ledge. Drop onto that ledge to find a crate. Open it to get three "Special Iron Ingots". The crate will only spawn if you do not have any Special Ingots remaining. When you use those up, fast travel to any location and the crate will respawn. If you have gotten all the chests, you will still need to go to the crate twice to have enough to make all the weapons.