help finding a battery replacement? We bought a Watseka XP Cargo Electric Bicycle off amazon back in 2014
recently is hasnt been working and after further inspection we found that one of the batteries has failed (the one pictured below).  However i have been unable to find a replacement.  any suggestions?  its form factor is quite different from the what is standard now (the existing battery is shaped like a cube, where as all of the new ones i see are rectangle).
http://watseka-bikes.com/ seems to be down and has been for a while now so i think the manufacturer is out of business
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.