Domestic pressure washers run at 3000 rpm. At this rate of speed, you’ll need to have a constant source of water. That means your pressure washer would need to be fed by mains water.
However, as your pressure washer gets bigger and the rpm gets slower, you can use another source. This is usually at about 1500 rpm. Here you can use a clean water storage container like a barrel. Or you can also use rainwater.
Keep in mind that it’s important to use clean water for your pressure water. This not only means cleaner water for hygiene (after all, why clean with dirty water?), but it is also better for your pressure washer.