I bought a still air, styrofoam LG incubator years back and added an old fan from a PC. As mudfarm said, you'll probably need a 12Vdc adapter (aka wall wart). Check the fan, it should have the power requirement written on the label.
From what I'm hearing, most of the incubators with air circulation only turn on the fan when the heater goes on. Mine stays on all the time as this provides more uniform temp. Plus, I didn't have to concern myself with connections to the heat controller.