Here are the ones I use:
http://www.1800airfare.com/ http://www.cheaptickets.com/ http://lowairfare.com/customer/ http://www.lowestfare.com/cgi-bin/gx.cgi/AppLogic+lowestfare.home.Home

Depending on where you go, each specializes in a different area. I was able to go from San Jose to Belize for $450 (although I can't remember which one of these I used) and Travelocity, expedia, etc. were showing $890.00. Consolidators are a great bet (some are listed above). Find one in your area that specializes in Mexico & Central America and they can get you a great deal!

As mentioned in other's messages, you can go to the airline that flys there and see if they have specials (Continental & American for sure)