You have to know someone from there, or book a place on an official tour. The Slovenian embassy or consulate in your country might have info about legitimate travel agencies which arrange them.
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.