24V DC Power supply on the cheap?
Its possible, but less safe, as these supplies are ground referenced, it means you have to deliberately disconnect 1 power supply PCB from mains ground (leave the enclosure grounded!)

This will then make that supply floating, and able to be used in series with another supply, again, it makes it more dangerous for yourself, so set things up so you don't have to touch it before powering it on. 

And to catch it before it gets asked, some ATX power supplies still have a -12V supply, however it is extremely low current and easy to damage, avoid using this.