Most cold air humidifiers have a resovoir pan which the water fills into. Then they have a filter which sits partially in the water, and draws water up into it like a sponge. Then they have a fan above the filter which pulls air through a vent, through the wet filter, and out into the room. Rather than buy one, you can build a system like this easily with a little creativity.
Hot air humidifiers use heating elements. they use lots of electricity and create bacteria and that a long time to heat up and humidify the air.
Ultrasonics are the best route. You can get a large plastic bucket, fill it with water, and put a pond fogger in the bucket. Voila, humidity. Just remember to fill the bucket up every few days.
If you want to control the level of humidity, get an electronic inline humidistat, Plug you pond fogger into one side of the humidistat, and an extension cord into the other side of the humidistat, and plug it into your wall. Program the humidistat to turn off the power at the desired humidity level. Presto. It'll turn on and off automatically.
You can find all that stuff at many hardware stores.