I would second the recommendation for V Australia. I generally find they are the cheapest for Australia to USA travel. The real trick with V Australia is to wait for one of their travel sales e.g. I have booked return tickets to Boston for around AU$1300 in the last year. It all depends on how flexible your travel date is, because it needs to fit in with the period of travel they are promoting. Other than that, I would be looking at STA, Expedia, Webjet and so on.
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