参考にしてください

http://www.airtreks.com/tools/TripPlannerIntro.cgi?ref=INDEPENDENT&dst=MAIN

http://www.independenttraveler.com/resources/article.cfm?AID=9&category=6

says:
We recommend Air Treks because they offer the best bang for your buck and have a Trip Planner which gives you the ability to create your own custom multi-stop international trip right on their Web site.