I bought the small clear plastic Vicks humidifier and took the center of it out. I carried with me for a time, but I got tired of carrying it.
Pediatric Warm Steam Vaporizer | Vicks | Walgreens
The blue middle part is removable and can be placed in the sink, tub, or some ice buckets.
Here's another one I found while googling:
Posts tagged humidifier - Crave - CNET Asia