If your wondering in Inotia 4 how to get leather it couldn’t be any easier. Leather can be found in any of the crates that spawn on a map. Enemies will randomly turn into crates on the initial load or re-spawning after being killed. It is also a random drop by just about all bad guys.
You can find some too at lamp merchants. The best way to get items from lamps is to save before using and if he doesn’t have what you need just reload your game.
The last and best place is in Improved Treasure Boxes and you can find a list of all their locations in the Inotia 4 Guide Wiki.