I did the same thing as your thinking... picked up 2 of the smallest fans available at memory express and hooked them into an AT power supply box that I had from a really old computer. Threw on a cheap timer and away I went. Worked well for my 10gal nano that I had running. I drilled 2 holes in the canopy on either end for the fans and reversed one fan so one side was sucking in air and the other was blowing it out.

Works like a charm.

you can mod an ATX power supply by jumping out two wires on the 24Pin plug, i cant remember which ones but i have done it before, but you are better off just buying a 12VDC adaptor like the ones that come with cordless phones etc,
i would give you one if you lived close to me as i have over 100 at work that we usually donate to schools.
you can even buy variable adaptors that you could use to slow the fan down, they are only $20 at circuit city and come with a range of plugs,
The best computer fans i have found are SilenX they can be tricky to find but NCIX carries them, by far the quietest fan i have ever used.

You need to be picky with this. There are two types of plug in transformers, regulated and unregulated.

If you use a regulated power supply like the Circuit City adjustable one it's easy but they are more expensive. No matter what the current draw is you will get 12v out of it.

With a cheap unregulated one like most of the ones everyone has sitting in a drawer you need to be a bit more careful. First figure out how much current you need for all your fans by adding up the mA rating on them. (eg. 4*150=600) and then go looking for a power supply that is rated at 12v and 600 or more mA. If you get one that does 600mA you're done, wire it up and go, however you are more likely to find one that provides more current like 1000mA. This is a problem because what that means is that the power supply will give you 12v only when you draw 1000mA, because we are only drawing 600mA the voltage will be higher like 15v. This probably won't set anything on fire but it will make them noisy and probably shorten their life.

The easiest way to fix this is to add a variable resistor or potentiometer (think dimmer) to the circuit to adjust the current draw and adjust the fan speed.

we sell fans that are 120vac, for around $30 and you would just need a chunck of cord and a male plug end...... thay are a 4"

dimmers are SCR's not Potentiometers just so you know, they dimm by flickering the power on and off, thats why you cant use them on arc lamps.
Most computer fans will be fine to run at 15-20V but it will shorten their lifespan.