There's two Lethia related missions and you have to do at least two separate runs of the Lethia Fleet encounter to get them both. Lava Ocean planets spawn a crap ton of Avian homes that have Avian related mission maps in them.

Otherwise, you have to get lucky with your space encounters and dungeon spawns to get all of the maps. Admittedly, grinding space encounters is better because you won't have to fly all over the universe to find them and can jump between a few already explored systems. There's one map that I've only found once for a mission called 'Highway Havoc' that I think I found in either a Penguin dungeon or space encounter. Not sure which.

thank you for the advice!