Go to your local computer market and pick up a computer fan (60, 80, 120mm models are all commonly availabe).
These fans typically run on 12V DC, but check the label on it, and it will often say. For the 80mm version, go out and buy a power adapter that supplies 12V DC and around 300mA of current. Cut both ends of the wire, connect them together and voila! I am too cheap to use proper connector caps, so I insulate the connection using a glob of hot glue, and then enclose a whole connection in a small plastic tube filled with hotglue (this is optional).
An easy source of the plastic tubing thingie is the plastic thingies you put into a drilled hole in the wall to put a screw in. Yes, my vocab for tools and hardware are very lacking!

You just cut them open and put the wires' "joint" in there, fill it with hotglue, then close it up again.
I just made one 2 days ago, can post photos if you wish to see them.