Season 3 Missions are also on Drive-Thru RPG. Seasons
1 and
2 are available on the SR Tabletop site. Season 1 is for SR3, Seasons 2-4 are SR4.
The current Missions (the ones you'll see at conventions) are for SR5, starting with Season 5. Currently only the first four of six Season 5 missions are available on
Drive-Thru RPG, with the remaining two somewhere in the pipeline. Season 6 continues from Season 5, some of the Season 6 missions debuted in 2014, the rest in 2015 at conventions. The 2010 CMPs (published as
Sprawl Wilds and
Firing Line), 2012 CMPs (published as
London Falling), and 2013 and 2014 CMPs (not published yet) are so far legal for SR5 Missions play. (Also see
here for a note from Bull regarding the Missions numbering scheme.)
You can try the
Catalyst Demo Team site, which has a searchable Event list. Also, you might do Google or Facebook searches for SR Missions groups in your area or check with your FLGS in case the local Missions GMs haven't posted events to the CDT site.