Assuming you haven't found an answer already, missions are exportable (and importable) as zip files. While editing a mission, you can use the export button in the mission briefing window to create a new archive under a KSP/Missions/_Exports/ folder.
To import one of these zips, drop it into the missions folder, and the game will automatically import it next time you open the play or load mission lists.
There is a community repository at kerbalx.com
, that features craft and missions for KSP as downloads. They also link to community mods that can make it as easy as using the Workshop for sharing and downloading missions. As with any mods, make sure they will run on the version of KSP that you are using.