How can I get the best deal going from Chicago to Las Vegas?

I want to go to a conference in Vegas March 19th and returning March 22. I currently do not have the money now to go, so it might be a week or two until I can pay for this, so where do I look for the best deal? Do I go on travelocity or those sites? I know I can get a flight and hotel deal package but how do I search this? Is going out of O'Hare cheaper or Midway? Can I get a better deal if it is at the last minute?