PC Fans
PC cooling fans are the best choice for mixture fans. They are brushless, meaning they don't create sparks which could accidentally ignite your fuel mixture, and they run on 12 volts, usually.
There are multiple ways to wire a fan to work in your launcher. A basic setup includes a 12 volt power source, screws to pass current through your (presumably) plastic chamber walls, and a switch to turn the fan on and off.
I personally like to make up battery packs from 9v batteries which provide 18 volts, and then use a 12 volt voltage regulator (check out RadioShack) to tame this to 12 volts. Two 9 volts wired in parallel form one "cell" of my pack, and these two "cells" are wired in series provide 18 volts and enough current to run the more demanding fans.
Also, a LED and resistor (or 12 volt LED ((LED and small built-in resistor)) ) can be wired in parallel with the fan, and in series with the switch, to provide an external indication of when the fan is running.
The red arrow is pointing to the 12 volt regulator, which converts the 18 volts provided by this battery pack to the 12 volts required by the fan and 12 volt LED.
This battery pack doesn't need to source too much current, as the fan doesn't draw much. The regulator will come with wiring instructions on the package, but basically this one is wired with the input on the ground of the slider switch. The center pin is the common ground, and is hooked to the ground of the fan, LED, and the negative terminal of the battery pack. The far left pin is the positive regulated voltage, which goes to the positive lead of the fan and LED. Simple.