Are there any NITs/IITs that provide final semister internship instead of project under mentor?
Yes there are many.Giving internship depends on the colleges and companies.Most probably a company starts interncship for a student in the 4th year.You may visit websites of IITs and NITs for internship related details.
Good luck.

Dear Nithin
It basically depends upon the college and as prabhavati Said I too agree with the points said by her, these days all comaplines recruits the students and will give the internships from 4th year onwards.
All the best.
This basically is not public knowledge. These days most of the companies recruiting are offering internships along with the placement offers or internships with convertible recruitment. It will basically depend upon the NIT / IIT and the companies that come for placement. One cant speculate about it.