Take it down to Ritchie street and most of the mobile servicing shops do that and definitely there is no reliable shop there. You need to bargain. The starting price of the battery cost around Rs.1500 excluding service charge and goes up depending upon the model. You can buy the battery online and fix it yourself. There are tons of YouTube videos on that and it is quite simple. I did that sometime back with the first generation IPad but it certainly does not give backup as a newly bought one. Maybe Apple has designed it that way that the older ipads/iphones needs to be discarded after sometime.