what voltage and current?
peak, not averages, as you want to interrupt a pulse
one place to look is the Grainger catalog
look for a Square D type box with at least a double pole double throw switch.
such as
https://www.grainger.com/category/e...witches?attrs=Contact+Form|2PDT&filters=attrs
but there has to be something less expensive and smaller
connect the wires from the charger to the center, connect the fence to one side,
and connect the other side to itself
the the switch will connect the fence to the charger,
OR
the switch will connect the two wires in the fence tape together
that should clear the residual charge.
You might also want to connect this to ground, so if there is any residual charge,
it has somewhere to go