Loads of them on Ebay and Aliexpress. Batteries have moved on a lot, so you can get one with 15Ah or more in the same case. They're called Silver fish batteries. Only two things to check. Firstly, that the dimensions are the same. The length doesn't matter much because they supply a new lock strip. It's just the front to back and left to right dimensions that have to be the same. Secondly, when it arrives, check the polarity of the contacts with a meter because there is no standard for which way round they are.