Under-volt the fans so they run quieter. If 12v fans, try 5-8V and see how the sound is.

Just repurpose old wall warts. Get a cheap dc/dc converter if they are too high. You can also buy adjustable voltage universal wall warts and dial in the voltage (fan speed) that you like. Make sure it has a amperage rating of 0.3 A or higher. Higher is fine. Lower is not fine.