The only simple solution is to installa wall mounted speed control, assuming there is an existing wall switch now controlling the fan. Shut off the power, remove the wall switch, should be two wires connected to the switch , and purchase and install a speed control.
Most have a rotary knob , similar to a light dimmer. Some come with a low speed adjustment, that you would adjust per the instructions , close up the plate and it will bring the speed down as slow as you would like.
Very important, is, you will notice, most speed controls always bring the fan on at high speed, then you can bring the speed down from there. It is not good for a motor to start and remain at low speed.
Following link is from Grainger and shows one speed control that I am referring to along with pricing: http://www.grainger.com/Grainger/wwg...605723&ccitem=