Cart Tek No Longer carries the replacement SLA Batteries. For your GRX-1200R you will need a 12 volt 35 ah deep cycle battery. We can recommend picking up a battery at a local battery store. We have had great luck with Interstate Batteries. Here is a link to their website: https://www.interstatebatteries.com/find-a-location
We have a drop in Lithium-Ion upgrade kit available Here.