I'm not english, so sorry in advice for mistakes. Some time ago i have made personal versions of some characters and i have used them for myself. I'm not an artist or a modder so i have made something like a "patchwork" with other's mods. If i want to post them somewhere i have to ask permissions to mods, have not? Ok. But some mods are very old and some artist are inactive for a while. So i don't know how to contact them. What should I do in these cases?
Your main options are... Contact the authors. Check their descriptions or permission settings on Nexus: If there's a link to source code or GitHub, check for a license file there. Unfortunately if none of those options work, you can't legally publish a mod containing their assets.
Is there a way to find an email somewhere? I thought I could at least post with a warning (like "If you want me to remove, I will") because i saw many people do that...
You can't get their email address so far as I know. Unfortunately copyright law doesn't really work that way (though some people might misunderstand or ignore it).