you want one that can auto-refill or be adapted to refill from a hose
You need to have distilled water for an ultrasonic humidifier to last (or very low EC - rain water)
I tried all those store bought humidifiers before...the cool mist ones (ultra sonic) would fail after a couple months and I went through several makers , and refilling was a pain ..the warm humidifiers are more reliable but you still have to fill them everyday..
http://www.glassstructures.com/products/accessories/atomizing-humidifiers
Get a fishtank air pump, some airline and an airstone. Get a bottle or bucket of water and run the air through it. The air breaking the surface for the water will aspirate enough water to increase humidity very effectively. This is how we would humidity air for lab experiments, running the air through a bottle with water.
It is cheap, give it a try.