What is the exact sources for downloading the ebooks.. actually I found many of such sites
But at end it asks for buying option.. now this time I need the exact result..

You want free e-books? Sorry we can't help you.

Books are under copyright, this is why you have to pay to own them.

Project gutenburg has over 17000 books that have expired copyrights for free:
Main Page - Gutenberg

There is no one source for e-books. Not only is there not a single source, but there are multiple formats not all compatible with all readers.

What type of books are you looking for?

It's just the same as buying the book in hardcopy but only in softcopy and if you want that for free then it doesn't make sense!