Which closest country to Nigeria can we find Slovenia embassy?

I don't think Slovenia has an embassy in Nigeria, but you could email their Foreign Ministry to find out the nearest one. They probably arranged for another EU country to handle visas for them, but you need to ask which one.

http://www.mzz.gov.si/en/embassies_diplomatic_missions_and_consulates_general/Africa/nigeria/ Don't buy an NSK passport. NSK isn't a real country. It's an art group that issues "passports" as a joke. You can't cross borders, get visas or live in Europe with an NSK passport.

You can read real info about the passport here:

http://www.nskstate.com/state/passport.php

Please ignore anyone claiming to be an "agent" for NSK. They are liars and will just steal your money.