A power supply is unlikely to be dangerous or deadly so long as you leave the cover on. There are some nasty voltages inside for the inexperienced but what comes out of them is generally pretty safe.
What voltage and current do you need? If it's 12V, an ATX supply will work, but it's also not hard to find 12V power bricks that are rated for 3-4A, sometimes more. They're often generically called "laptop" bricks or chargers even though the ones made for laptops are typically higher voltage.