help finding a battery replacement?
SLA batteries are all the same, 2V/cell nominal, mostly 6V and 12V batteries. In this case you have a standard 6-DZM-9 6V, 9AH battery. All you need to do is find a replacement 6-DZM-9. The best solution would be find a battery specialist (not just you corner car/truck and motorcyles battereis seller) and see what they have in their catalogue.  Ideally you want a Deep cycle battery desoigned for electric scooters, bikes carts etc, not a Standby power battery. The latter is designed for UPS with the very occational deep discharge and won't last quite as long as one designed for regular discharge/recharge cycles. 

With lead acid batteries, do not replace just one battery out of a bank, you will need to replace all the batteries - at 6 years old theyare at the end of there economic life anyway, and replacing just one battery will mean the life of the new battery is significantly reduced.