I doubt you will find anything cheap considering the power and number of outputs of some ATX supplies.
You'd basically need at least 5 CC loads (+5, +5VSB, -12, +12, 3v3V) and possibly many more, like for example
800W power supply can have
six +12V rails, of course you cant load all of them to full power but still that's 10 outputs in total to load.
So either DIY few CC loads, put it in common box and drive with some arduino, or just buy a bunch of remote-control-capable loads and control them from same place
.