I need to download shapefiles for all country from which i need to genereate topojson for around all countries.

currently I am using data from naturalearthdata.org which only provides Admin 0 ,1 for all countries. I need for Admin 2 ,3 which cover towns/cities/districts level details.

I have tried diva-gis.org i had to download country by country from dropdown which is taking lot of time.

1 . Is there any site i can download shape file for admin2,3 for all countries as single shape file or country specific shape files .(free/open source)

PS:I will integrate this map/ topojson inside d3.js /java /j2ee project.