12v 40mm X 40mm brushless fan.
Surprise! If you have bought a bundle from me you can get some more power out of your steppers by turning up the pots and keeping them cool with a fan. The drivers need a fan to push more power and stay cool and here is the fan to do it. Wire these directly into the 12v power plug, not the fan port on the board.
Quick and easy mount, http://www.thingiverse.com/thing:145946
Or a perfect print fan, either way you probably want one or two of these.