I made my own and it is very easy. Here is what you need:
12v DC adapter (i used one lying around from something else) - Free
12v PC fan (I used a 70mm fan I got off newegg) - $4.99
Project Enclosure from radio shack (I think mine was 6x4x2) - $6.49+tax
Ideal 32 Red In-Sure 2-Port Connector from home depot (and some electrical tape is recommended) - $1.79+tax
1" hose or pvc - prices vary depending what you get
You will obviously need a hole saw or dremel, something to make a hole on the front around 2-1/2" and 1" on side.
First thing you need to do is splice the wires of the fan and DC adapter. You can use this website
to help figure out which are the negative and positive wires. Connect them to the negative and positive on the DC adapter respectively and test it to make sure it works.
Drill big hole for fan and screw in, making sure fan is the right way (pulls air).
Drill small hole on side for tube.
Drill a very small hole on another side and feed the dc adapter wires through.
Inside the box use the 2-port connectors to connect the wires, and use electrical tape to make sure the stay on.
Then just re-attach the back plate, plug it in, and put the tube where you want.