Where can i find really cheap flights?

Where can i find really cheap flights?
I need to find cheap flights soon. The departing location would be Dulles International next to Washington D.C. and the arrival location would be Austin Texas. I looked on a lot of websites but the cheapest one is $300. My friend moved there a few days ago and he said he only payd $200 but i cannot get hold of him to ask him how he got his so cheap.
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