To slow the fan speed down, just reduce the input voltage. It won't harm the fan, if it's a DC fan. I use a power supply that has multpile outputs. 5V, 12V, 24V. You can get a power supply like this at Radio Hack, or partsexpress.com

If the fan is a 24V fan, run it at 12V, if it's a 12V fan run it at 5V.