For my humid hide I use a six inch plastic flower pot. It has little holes for draining water at the base. I cut up some sponge pieces and hot glued them to the bottom over the little holes. Then I cut an access hole and flipped it upside down. I can then squirt water at the little holes, which re-moistens the sponge. Works pretty good and didn't require a trip to the store at all. Just need to wash it well once a week or so for prevention of mildew or mold. Hang on, I'll post a pic.......................

Not very pretty, but works well.