The safest way is this:
1. look at your fan's motor and see what amp it requires. The 80mm fans usually need around 100-150mA.
2. Go to a electronics shop like RadioShack and purchase a "regulated power pack" that has a amp output >= your fan. The close it is to your fan's need, the cheaper it will be and the cooler it will run. IF you already have a UNregulated power pack, and you wish to wire that, then it needs to have voltage output > than the fan's requirement. Then solder a regulator into there to run the fan.
The matching of the amp output doesn't have to be spot on. If the PSU outputs more, you fan just spins faster. However, if it outputs too much, then it will just melt the fine wire coil within the motor.