Best place to start is probably Mission4Today
http://www.mission4today.com/
Click on the Downloads Center.
There is a section for stock missions/campaigns and a section for modded missions/campaigns
All of the stock ones will run with HSFX6 however some of the modded ones may require other mods or mod packs to work. Read the notes and ensure you have all the maps and objects required before loading a modded version.
You will also find many posted here
http://www.sas1946.com/main/index.php/board,111.0.html