Most Quake 3 characters will work. Just download 'em from sites like Planet Quake and then put the .pk3 file inside the BASEOA folder. Your character should be available from the model select screen (OMG secret button).
EDIT: Forgot to add, even though they can be used, bot support will not work as intended, so I advise you to either just use the model yourself (or your human friends
), or edit the bot files (if you know how).