They either suction to the cage or sit on top. Gravity takes hold and the water runs down the hose. There is a valve that you turn to adjust the flow. If you think about it all you need is a soda bottle, aquarium air line, and a aquarium air line valve.
Its really easy and thats something right off the top of my head. Im sure with more thought you can think up a cleaner cut way of making one. Well a canister for the water anyway. The tube and valve are seriously just cheap aquarium stuff.
Proper misting should be sufficient however anyway.