you can get the processor + mobo at Sony computer or ETC at Canada Corner. Also you can try Rahul traders at MG road.
The prices for the processor + mobo is almost the same as at lamington road. But getting the right gfx card and psu will be a problem, you are better off ordering them online.
PS : If you go to Sony Computers just tell them what you want, if they dont have it dont bother chatting with them ( unless u want a good laugh) as they are the most ignorant lot i have come across!!