what L1ftmech says is true, you can make your own battery pack using automotive batteries but in some cases some of these electric trucks are even sensitive to the voltage and amp draws and a homemade automotive battery pack may or may not work.
If your going to build one from 3 12v batteries use deep cycle batteries, they tend to work better in that type of application. Group 31 size batteries would probably work fine for this.

I know they are bigger and heavier but you will have less problems in the long haul. You can always build a rack to put them on with a hook that would hook onto the lift bracket and yet still be removable.

just a thought