Either a heat bulb or ceramic heat emitter will work. They do dry the air but if you have a cover on the tank that will help keep the heat and humidity in. You can also use two water dishes - one on the warm side, the other on the cool side - and the evaporation from the warm side dish will help bump up humidity.
Rather than try to make the whole tank humid you can also make a humid hide. Get a food storage container that is just big enough for your snake, and cut an access hole in the lid. Put about half an inch of damp eco-earth in the storage container, put the lid on it, and that can go on the cool side of your tank. If you're having problems with bugs and mold in the eco-earth you can bake it in the oven to kill the spores, bugs, and eggs (make sure you take it out of the plastic first) before you use it.