Getting polygons of all countries and ISO-3166-2 subdivisons

This site may have what you are looking for:

• http://www.gadm.org/ and http://www.gadm.org/country

You can find a script inspired by MvG's answer and a few examples here : https://github.com/laem/regions-topojson

Sadly, not all geonames administrative relations are tagged with iso-3166-2...