Any computer store would carry them, just tell them you need a computer case fan. You will also need a computer power supply to hook it up to, and you can find it here:
older style 200W CSA power supply, non-standard shape, $7
Newer computer power supplies will not work because it needs to be plugged into a motherboard before it will power up. Older AT power supplies do not have this safety feature.
If you use helix thermostats, you could get this too, but it is really pricey for the same funcitonality.
Another option would be to hit Home Depot/Walmart, they should have similar fans that you can simply plug into an electrical outlet.
Hope this helps.