If you're looking to replace the battery with the same lithium polymer cell, get in touch with Justin because he is in contact with the factory that made the lithium polymer cells that are inside the Zero. I did a search and wasn't able to find off the shelf alternatives. This is because the factories will customize the dimensions of the battery to fit the application.
If you're looking to replace the battery with a higher mAh capacity cell that lasts longer you probably won't be able to fit it inside the Zero. Since the battery is one lithium polymer cell, the maximum voltage is fixed so to make the batteries last longer, they increase the volume of the cell. The total volume of the lithium polymer cell is directly related to the capacity (mAh) of the battery.