Where do I get used 11th , 12th ncert books for free ?
You can go through with the given below link, This online service offers easy access to the NCERT textbooks. The service covers textbooks of all subjects published by NCERT for classes I to XII in Hindi, English and Urdu, You can choose the class, Subject & book name also with given link.
http://ncert.nic.in/textbook/textbook.htm
Good Luck!