The power supplies are the things you plug into a wall outlet and plug your cell phone or other DC device into to charge. Some people call them wall warts. At Radio Shack they are kind of expensive starting at like $15 and up. I haveseen them at Frys Electronics in the variable voltage variety for between $6 and $50 or so. I would post a link to the ones I have from Harbor Freight but they are not listed on their website anymore.
Here is an example from a different thread:
http://www.allelectronics.com/cgi-bi...E_OUTPUT_.html
DC fans have a 4 wire plug with 3 wires in them. You only need the red and black wires, ignore the yellow wire. Cut the plug off, strip the red and black wires back and do the same with the power supply as it only has two wires too. Twist them together or use a crimp connector or wire nut and tape it up and you are running. 120mm fans are the standard 4" fan size used everywhere.