I travel a lot.
Normally I search for flights on two sites:
Mobissimo Yahoo Travel Farechase
Both are good in North America. Internationally Mobissimo is better.
Recently Mobissimo has added a killer feature – Activity Search. One of the choices is Trekking.
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I was shocked at the excellent search results from my city, Calgary. These guys know trekking!
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I shouldn’t be surprised.
Blogger climb_ca on the Getoutdoors.com Outdoor Blog GoBlog is associated with Mobissimo.
besthike.com 
Give Kayak.com a shot.
From the travel guy @ http://www.traveldealspotter.com
Very interesting information. We travel a lot within the US and usually use a direct website for airfare and rentals. This might be a more reasonable way to go in the future. Thank you for the links and I will explore…
Another vote here for Kayak. I use it when booking flights as well.