Good place to look for airline deals?

I am looking for airline tickets to Las Vegas for the dates of August 23-August 27. I am willing to leave anytime, although I would pay a little bit more to leave early morning on August 23. I am having a hard time finding anything under $260 roundtrip. I took this same trip in July of 2008 and paid $180 roundtrip. I am willing to leave from a number of places: Madison or Milwaukee, Wisconsin or anywhere in Chicago/Rockford. I am a very frugal person but I am not very experienced with finding cheap flights and I would appreciate any advice! I already found great deals on hotel/car rental and I would love to keep the price of this trip to a minimum!