Assassin's Creed Valhalla Leather and Iron can be found in Minor Caches of Wealth scattered around the world and purchased from vendors.
While you can use the Raven of Eivor to locate iron deposits and animals for leather in Assassin's Creed Valhalla, the amount earned for each is not much. You should still break open iron knots when you encounter them and kill animals stupid enough to cross your path, but there is an easier way to accumulate a much larger treasure of leather and iron.
You may have noticed the three orbs of different colors that fill the map of Assassin's Creed Valhalla. Yellow indicates wealth, but there are smaller yellow orbs on the map that do not contribute to area completion. Zoom in fully on the map and these orbs will appear, and you will easily be able to distinguish them from the larger items of wealth due to their much smaller size. These are chests, and you can locate them by pinging your Odin Sight. Keep in mind that there are chests scattered around Christian monasteries that Eivor and his crew can raiding, and these tend to contain Settlement Improvement Supplies, not Leather and Iron. .
Each chest usually awards 50 to 60 of the two materials, so whenever you pick a clean area, it's best not to ignore them. You can also buy Leather and Iron in Assassin's Creed Valhalla from vendors. Each vendor has their own stock and each carries 200 leathers and 200 irons. The stock refreshes after about an hour of real time and the exchange rate is silver for iron or leather. In other words, you can grab 200 pieces of leather or iron for 200 pieces of silver, which isn't terrible. You'll want to prioritize chests if you want to keep your wallet, but vendors are great for supplementing your supplies when you need a few more pieces of leather or iron to complete an upgrade.