Sorry... I totally edited my post right after you (apparently) read it, because I noticed you already had the repti-temps. So anyone else reading this now might be confused by your response, LOL.
They do make humidifiers for small terrariums, as I remember seeing them at a reptile expo - I think they were made by Exo-Terra?? Doesn't seem many people use them for ball pythons, however, since it might get things toooooo humid. Have you considered switching them to tubs? I had major humidity issues before switching mine over, and now I can't keep it LOW enough!Before I go off an buy those heatpads you have linked to... anything out there would work as a small terrarium humidifier? not sure if anything like that exists?