Last 5year Papers for Jee Advanced for admission to IITssharedbelow. 

https://engineering.careers360.com/articles/jee-advanced-question-papers

For Mtech you shall need GATE questions papers as below for download  

http://gate.iitg.ac.in/?page_id=59

There are many books which have a collection of Previous years IIT Question, Many publications Including Arihant have their books which are available online You can Buy them.

At the official site of JEE Advance, You will be able to download Previous years Question papers.

You can follow this link to get questions from 2007 to 2018.

https://jeeadv.ac.in/archives/past-question-papers.html