Look through the Character Modification threads for links to the skins. There is a tutorial somewhere in there on how to replace the original character skins, but i have never got it to work correctly. Other forum members could help you more with this...
To extract/replace files you will need to get AFSExplorer which you can get here:
https://www.moddingway.com/downloads/9b ... er_3_7.zip
You will also need the NBD Tools which you can get from http://obsrvfun.nusutto.jp
website. Pm me if you can't find them...
Here are some tutorials and other info about swapping/changing the models:
Tip: Use Google Translate to read the info in your language on those Japanese webpages
Gotta go now. Hope this helps to get you started