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
<resourcepack.zip>\textures\entity\enderman\
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.