buy a whole goddamn transformer
A computer Power Supply generally has a transformer in it, depending on the design.
So just buying a transformer would not work out well for you since the voltage would still swing from +12V to -12V. This would cause your computer to release all of its magic smoke.
You also need a rectifier to convert the AC signal into a DC signal as shown below
But notice how the signal still has valleys in it. If you hooked this up to your computer, it would still explode, or at least reboot 120 times a second.
Then you add capacitors, inductors and other items to "smooth" out the signal.
However, computer power-supplies are generally Switching Power supplies, which allows them to be up to 90% efficient. If they instead were a linear power supply as a was showing above you would struggle to get 30% efficiency.
As for a Computer power-supply that is ~650W and ~$90 I would recommend either:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703036or as Supertoaster suggested:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005