Only moments ago, I was researching these devices, both the fans and the ultrasonic foggers. The foggers are 24vdc and your choice of fans should be based on that figure. I have a number of 24vdc muffin fans, about 50mm diameter which were removed from discarded workstation computers, but that may not help you, other than to indicate that you should be able to find a fan of this voltage. You'll be cutting into either the wiring at the power supply lead or into the wiring at the ultrasonic unit, in order to join them, but other than that, it should be no serious challenge. The power supply for the ultrasonic humidifier should not be rated to be maxing out when using the unit, so there may be enough "suplus power" to run the fan. That's only item about which you should be cautious.
A quick search on eBay shows dozens of low priced 24v computer-type fans and an equally large number of low cost ultrasonic devices.