You can buy 'em dirt cheap at any computer store. They start at around 5 bucks and top out around 20. Rat Shack sells a lot of low quality products. You'll find something much sturdier and cheaper at a computer store. The power supply is usually on display directly below the fan you're looking for.
If you're cheap,you can go the online electronic supply house route. I've seen them run 3-5 bucks there before shipping on a regular basis.
Ham radio and computer fests are the best place to pick them up,IMO. You'll run into nice heavy duty ones that were built to cool tube transmitters and amplifers. People practically give them away. Do a little digging through the vendors junk boxes and you'll likely find a power supply that will work with your fan. We're talking under 5 bucks for the whole setup.