Talismoon released their "PSU Energyzer for Xbox360". With this product, you'll be able to use the PC atx power supply of your choice on a xbox360.
The cable adapter is 21$, and you can find a pc power supply at 10-14$ easily.
The PSU Energyzer is a special cable with an OEM Xbox 360 power plug on one end, and a 20 pin ATX connector on the other.
Please note that the original Xbox 360 Power Supply provides 203 watts, and 12v DC current at 16.5 amps, and 5v DC at 1 Amp. For the PSU energyzer to work, your PC power supply must provide this much SUSTAINED power at a minimum. Many PC power supplies are rated for PEAK power instead of sustained power, so you must check the label of your power supply to see if it provides enough sustained power.