I found one that is plumbable and is carried in the baby section at targets everywhere. It has a round opening that is 1 7/8ths wide. It is $30. I used a rubber stopper that is about 3 inches wide at the top and 1 1/2 inches wide at the bottom with a hole drilled through it. then I shoved a 3 inch long 12 mm glass tube through, wrapped the end with black tape and shoved it in a 5/8ths clear rubber hose. It works super great and is easy to remove for cleaning. You could probably find that stuff at the hardware store, if not glass a metal tube would work just fine.
Safety 1st Ultrasonic 360 Degree Humidifier : Target
I had a question though, should I paint the reservoir and rubber tube black so they dont grow bacteria and mold? I dont want to breath contaminated air...
EDIT:
A word to the wise: Make sure if you are going to use one of these that you position the hose so that it doesent sag, because water will fill the low point and block the flow of vapors. This also causes the machine to get very hot. Which im sure will cause it to burn out faster.