Back to the original question. Fan and power supply cheap and easy.
If you are going to use 12-24 volt fans i.e. the fan used in a PC I would recommend also using the power supply from an old PC. The power supply will have various outputs +5V, -5V, +12V, -12V anmd possible +3.3V. In various configurations you van have 5V, 10V, 12V, 17V and 24V which you can used for various speeds on your fan with the addition of a switch.
If you are going to use 12-24 volt fans i.e. the fan used in a PC I would recommend also using the power supply from an old PC. The power supply will have various outputs +5V, -5V, +12V, -12V anmd possible +3.3V. In various configurations you van have 5V, 10V, 12V, 17V and 24V which you can used for various speeds on your fan with the addition of a switch.