If you plan on modding an ATX power supply, don't rely on the -12v and -5v outputs. -5v is not even in the current ATX standards, only very old designs of power supplies still have it. And -12v... well, usually you only have 0.1-0.5 A of power on that, very little.
Modern ATX power supplies have little ripple on the outputs, but still much higher compared to linear power supplies. While a linear power supply can go below 5mV ripple, most ATX power supplies have around 40-100mV ripple on the 12v output.
Really, a small +/-15v 1A transformer is about 12-15$ shipping included. If you don't need positive AND negative, transformer will be even cheaper.
All you need after that is a bridge rectifier (or four diodes), a few capacitors and a couple of adjustable linear regulators and some resistors to set the output voltage. And if you want to limit current, that's also very simple to do, but you're already limited to 1A by the transformer or the linear regulators you're going to use.