Whats the best place for cheap plane tickets?

Flying to Vegas for work training in late July. I’m staying after the training and turning it into a mini-vacation since my flight is on the companies tab, but my girlfriend is flying out to join me after the training so we’re going to split her ticket. Whats a good place to find cheap tickets online, or alternatively, which would you recommend (since we’re probably all familiar with the basic names; Travelocity, Orbitz, etc).

Located in Columbus Ohio if that will make any difference.