How can i find a cheap Airfare from Australia (Perth) To US (chicago), I am not in a rush, and can travel any?

Try bestflight.com.au also kayak.com are good because they show you what the fares are in a little calender on the side for the month you select. If you find a flight that is cheaper than on bestflight.com.au give them a call and they might match or do better.