are you talking case fans or the processor fan? case fans are a piece of piss and only cost between £1-5 each depending on what make you get.

If its processor fan then you will need to buy a heat sink aswell as the fan, the heat sink is the metal block that sits under neath the fan

case fans http://www.ebuyer.com/store/Components/cat/Case-Modding scroll down abit.

If its the processor one then let me know as ill need to know some more info before i recommend one