You can buy one here
I was waiting for my battery to die in order to buy one but the f$%^ing thing still working after 4 years of use, this year i will change it anyway because i want to reduce weight, i calculated that a set of new pipes and this battery will remove 7 kg! from the bike and this is allot a specially when this wight seats high up.

you can build your own battery, the sells are from the company A123 and can be bought separately the only problem is the soldering, if you play to much with the solder it will destroy the cell so you need to be fast and that means good equipment.

From what i am reading, there is a lot of happy riders with this battery