Abolisher's Mission Command Outpost
is a good place to start for missions. Granted, they're on the Empire's side, but Emperor's Hammer
has a wealth of XWA missions...downside is that you need to be a member to access them. The Rebel Squadrons
, Emperor's Hammer's Rebel-minded sister organization, may also have XWA missions, but again, you have to be a member.
You might also think about getting the registered version of AlliED
, Troy Dangerfield's exellent XWA mission editor...it can convert missions made for the older games (especially XvT).
You probably should direct all future XWA questions to the XWingAlliance.net forum, as we've gone way off topic here.