Mallorn nuts are obtained from the mallorn trees of Lothlórien. They have a small chance of dropping upon breaking mallorn leaves. In this way, they are harvested similarly to apples, but are slightly rarer.
They can also be found as loot from chests in the treehouses of Lothlórien.
Curiously enough, they cannot be used as seeds to grow Mallorn trees, instead you need a sapling.
Food of Middle-earth
Banana Bread •