5v, 12v, -12v from 12v, 5A supply
To get all the voltages you need from just a 5 V USB or an adapter, I would use this board



Input: 5 to 24 V DC

Output: + 12 V, -12 V, + 5 V, - 5 V, +3.3 V
maximum output current: 300 mA per channel

You can buy these are a DIY kit or soldered and ready to use.

The design is based on a Cuk converter.

I have one and it does the job.
Much safe than an ATX supply as it cannot deliver so much current that wires will melt. It's short-circuit proof.