I see your location as Kolkata. If you have plan to buy a pre-configures model, then you can buy it from any tech shop in Chandni. Otherwise, you will have to go to exclusive Dell showroom, which happens to be near Kakurgachhi, CIT market.
i spoke with dell. They sell only pre configured machines, not according to the customization of the customer. Now, dell india site is the place where the vostro 3750 ia available with i5. In USA ony i3 is available. However in techtree.com they are showing that an i7 version is available