There are Skins, and then there are mods.

Skins for characters in game are usually Server specific, because they're modded servers with custom and tuned options, from the Admins side at least. Theres a way to pull up key commands in game to choose options within these modded servers, but these servers are far and few. Almost usually you can only few character skin option key commands on the modded servers website. (which is booty to me, there was a modded server i came across quite awhile ago that you could choose a modded skin via bringing up the character key command list, but sadly i dont recall the name or even have the server in play history).

And the mods to download and use ingame.. You probably already know about those. lol!

Hope this helps a bit. :)