Agreed with the above, but I'll add two quick points.
1) Make sure that your 9v to 12v (or any voltage in between) is rated to give at least 500mA, preferably 750 or 1,000. If your fan has a current rating given on it, just make sure your power supply is than capable of supplying the needed current. Some 9-12v power adapters are very low yield, most aren't.
2) Don't fret polarity. If you wire the fan backwards, it just blows backwards. No harm at all, just orient the fan to blow whatever way you want it.
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