Also available on DriveThruRPG that I could find:
The CMPs (Convention Missions Packs) are missions designed for play at conventions, but they're slowly being compiled and published and they're basically like the main SRM line except they have their own story arcs. (I have no hard info, but I can only imagine there's a plan to print them all eventually.) A couple other CMP compilations (Sprawl Wilds and Firing Line) came out in softcover only at conventions; someone might still have one for sale but it may be hard to find.
Other than that, you'd have to play at a convention or find a nearby CDT agent to run one that has been run at conventions but not released. All six Season 6 adventured have debuted at cons (and 8 CMPs per year since 2013), but only the first two Season 6 missions have been put up for sale so far. (We're working on them!)