I've never bought one myself, its always been an included item. Perhaps once you have the battery try Jaycar (or maybe Batteryworld) to ensure you have the right connector etc. mentioning Batteryworld, they do sell all sorts of proprietary style items, I bought a battery for my Dyson vacuum from them, but its probably a long shot that they do anything for Icom.
Incidentally the repacking option suggested by others is certainly a viable one, just copy the existing connections meticulously.