Planes not already accepted in the official FlightGear repository are not hosted on the FlightGear Web site, but on external services such as GitHub or Gitorious or on the personal Web sites of their developers.
Should you not find a plane, you can
search for it in the wiki, its description page usually has a download link.
In this case, the Dreamliner is
hosted on Gitorious - click the "Download" link at the top of the page to download the aircraft.
Note that, as stated in the README, after uncompressing the .zip file you must rename the folder "fg-boeing-787-8-main..." to "787-8", otherwise FlightGear won't load the aircraft.