Minecraft stores skins for all creatures in Minecraft as .png files.
These can be found in the .Minecraft folder, under the "resourcepacks" sub folder. You can open each one with a program like WinZip, 7Zip, WinRAR, etc. You can also find this folder by opening the Minecraft launcher, choosing "Resource Packs" the "Open Resource Pack Folder".
The contents of each holds a file for each and every texture in the game, including the textures, or "skins" for each mob in the game. The enderman skin can be found under
However, if you're looking to change the actual model of the mob, this can only be found in the Vanilla Resource pack. The Official Minecraft Wiki has a tutorial for Editing these.