Building a peltier power supply is actually pretty simple.
http://www.overclockers.com/tips1054/index07.asp
PSUs for PCs are not good for peltier cooling as they are designed primarily to keep voltage from fluctuating and have good rails for very quirky silicon stuff. They are not designed to run 300+ W of peltiers off the 12v rail, especially since the amperage drawn from the PSU will probably kill it. Also, much of the power created in a PC PSU goes into rails not suitable for peltier stuff. I know a large portion of the amperage for my antec true 430 goes into the 5v line, where it is needed in a AMD computer.
If you want to save the hassle of building a powersupply and throw your money away for nothing, go for the pre-built one at dangerden.