I recently found out you can get foaming soap refills as tablets. I hadn't previously found a way to refill hand soap pump style dispensers without more plastic. These are from Meliora.
I refill with watered down hand-soap (about 3 parts water to 1 part liquid soap). The pump bottle is what makes it foam, it doesn't have to be a special surfactant, just anything that is low viscosity.
Be careful not to refill above the level of the internal pump - if you do it can get flooded and the extra pressure can break the pump mechanism.