I heard from some people that Rescue 15 is providing locks that are really strong and good. So I called Rescue 15 Islamabad and inquired them about the lock. He said that currently we are out of stock but I can give you the telephone number of the company which provide us locks.
I called that company. Its name is "Watch and Guard". Its lacated in F-6/1, main Nazim ud Din road. So I went there. The price of the lock with normal size padlock is 1600 Rs, and the price with larger padlock is 1900 Rs. I bought the one costing 1900 Rs. It seems really strong. As it is a mechanical lock so it can only be broken by applying force (not by tricks as in electronic systems). But operating it is really difficult. You have to open the door and sit on road to lock it.