if you are ally then i would say duskwood is the best place i farmed it from the warewoves that are lvl 30,31, i think you start getting constant medium from lvl 25+ mobs
Try typing "medium leather" in the search box to the left - it will tell you which animals drop it, their level, the average drop rate, and where they are located. Then pick a zone you're comfortable grinding in and kill the appropriate animals.