Looking for it I had no results, also the manual is unclear how the batteries are placed. But I came to this:
POWERIZER 2600mA Tx BATTERY 4 JR SPEKTRUM DX3 DX3.0 / 2608B-9V MADE IN USA | eBay
In that case you should have a tray like this:
If that is the one, you should be able to find one at a local electronics shop.