Leather is a fairly common resource.
On the map
Leather can be picked up by a lord directly from the map. Leather is represented by a pile leather rolls on the map.
Leather can also be obtained as reward from most quests. The first quest that offers leather as reward is Investigate the forest. The first repeatable quest that offers leather as reward is Kill kobolds.
You can purchase leather from a level 2 market by using gold.
Workers assigned to work in a leather farm produce 8 units per season.
A lord can transport in its inventory stacks of up to 3000 units.
Leather can be sold in the market to obtain gold.
Depending on building type and level, some leather might be required. Most buildings require leather.