If you want to test this on your own, a few ceramic power resistors should be enough. Calculate the load with Ohm's Law. You obviously want a high load, so choose a low resistance. The specific value doesn't matter as long as the load is below the max current for the rail AND for safety is within the rated wattage of the resistor. A 10 ohm, 15 watt resistor for the 12V rail is good for example, drawing 1.2 amps.
Ideally you could add a 12V fan across your resistors to help manage the heat. A few fans would also provide a load for testing.
Typically you only need to load the 5V or 12V rail for all the rails to be stable or for the PS to turn on. It's normally whichever rail is the higher capacity one. Older PSUs tend to be 5V while newer are 12V.
That said, you can buy ATX load testers online at the usual stores that already have everything you need to load test an ATX power supply.
