Here one can find the Intl. rates by India Post, all in a table:
http://www.maharashtrapost.gov.in/htmldocs/internationalchart.htm

we need the API / documentation for making the module, but didn’t find the same any where

I am not sure to what your refer with API
but I am sure the only thing you can get from the India Post…
are those simple tables! ;o)

I am happyto hear your want to develop a module:
India being so big, you will find lot of people wanting it, I guess.