I had checked in Airtel stores and they don't stock it. Found a battery at a local shop. It's available online too on Amazon. Though the quality of the battery that I got and also the ones on amazon is questionable. I'll be running it mostly connected to a powerbank and since it doesn't boot without a battery I had to find one. The OP had already unlocked the device and was using it with JIO sim. The OP had forgotten to reset it before shipping and didn't remember the pass to connect to the wireless n/w. Fortunately he hadn't changed the admin password from the default one and I was able to access the admin panel by connecting the router to a windows PC via a USB cable. There's a hack online which enables running the router without a battery though haven't tried it yet but am planning to.
@babhishek I don't understand. Do you mean taping the battery terminals and inserting the battery in the router? Does this allow the router to function without charging the battery?please keep me posted, taping the battery also ensures battery acts as passthrough, no charging