I also refill all my foaming hand soap dispensers. I buy a bottle of Ivory dishwashing liquid and use it in my bottles. It is concentrated and mild on hands, so it is perfect. It only takes a small amount, maybe less than a half inch and then finish filling with water. Shake gently.
When the bottle quits working well, I pitch it and buy a new one. I like doing this because it saves money, is not antibacterial, and one bottle of dishwashing liquid makes many refills.