Will second the posts about purchasing water at the little grocery spots in DVC resorts. I've done it many times over the past decade.
Also, when flying with one, I have never contacted the airline in advance and have never had it questioned. I've also traveled internationally with it without any issues. We went on a AMA Waterways River Cruise on the Rhine river and the cruise line gave us the water, no charge (but then they charged enough for the cruise

).
Also, in a pinch, you can use tap water if distilled is not available. I've read that you don't want the humidifier tank to dry out so you don't get any residue build up. When I have done this, I've emptied out the tank daily and wiped it dry.
Hope this helps.