There is a lecture in our Healthy Spirituality course that discusses some options for removing Fluoride, as well as other impurities, from water:
http://gnosticteachings.org/download/healthy-spirituality.html
Check out the audio and slides for lecture number 6.
Just as a note, that lecture was given several years ago, so some of the products may have changed since then. It's always good to do your own research before making a purchase so you can choose an item to suit your individual needs.