I'm not so sure about using dry-ice. The intense cold might trap moisture by converting it to ice or liquid that can't be pushed out by the sublimating CO2.

Dry ice is pretty easy to get. You can buy it here by the pound at many grocery stores. But, you can also get CO2 from most any welding supply house. It is pretty cheap. A simple rig with a rented tank, regulator, and hose would be easy to use.