The energy comes from the surrounding atmosphere. When the balloon moves up, it displaces the air above it. This air has to be transferred to the space below the balloon. This air is losing its potential energy. For every unit of volume of the balloon gas going up, there is a unit of volume of the surrounding air going down. The gas in the balloon is gaining potential energy, but its density (mass per unit volume) is less than the density of the air that’s losing potential energy, so there is a net decrease of potential energy. This difference of potential energy is converted to the kinetic energy of the rising balloon. Some of it is also dissipated, if the flow of air is not laminar.