After a bit of hunting around I found an eBay seller who had replacement batteries for a reasonable price. I ordered it on Tuesday, it arrived today.

design capacity: 25940 mWh last full capacity: 25940 mWh

The original battery lasted over 2 years before needing replaced, so hopefully this new one will give me as good service. If not, I now where to get a new one πŸ˜‰