There is a direct flight from Bangkok to Changsha, but it is only be available on Thursday, Friday and Sunday. No direct flight is available from Changsha to Los Angelse, you need to connect the flight via Beijing or Guangzhou.
You can contact the flight booking center in TravelChinaGuide to help you. The eamil address is travelchinaguide.com|flight. You can also contact them via MSN: travelchinaguide.com|hotelbooking