Dear mehta,  

It is depend on where you want to do internship. If it is in a research laboratory like SPL, then I think you can contact one of the scientists there. In other centres like LPSC or SDSC, it is more difficult. I work in LPSC and I have seen very few interns (as in project interns) there. 

If you want to be a paid apprentice, things are easier. ISRO centres recruit them all the time. However, it will be difficult for an apprentice to do a focused project.

Thanks for asking here.