In general you want secret missions that either let you rescue someone directly, or gives you a rescue survivor power that will spawn rescue missions on the map for you.
If you want them ASAP I've found that the skelebot junkyards are the easiest as you can 90% of the time just pop your head into the shacks and see every room. If a room is orange, check it out. If it's not, try the next shack.
Don't go through maze rooms obviously as they are too big to explore quickly. I don't recommend caves but they can be down there. They can be in any building it's just chance really.
The normal missions also reward rescue powers sometimes, so look for them.