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to the best of my knowledge lightning is wind mastery and if you look at each of the regions they often say what magics the races specialise in so if you recruit from those races then you have the best odds 

Drow are the most likely to have dark magic available, so if you do some recruitment missions in the tunnels then you're likely to get some 

From what i can tell there are 2 (3) ways to get Air Mastery 
1. a rare mission in the city will let you recruit a air master if you crit but its difficult 
2. if you have a air user and some air user slaves a mission will pop up after some time saying you can infuse your slaver with the power of the slaves magic upgrading them while also stripping the slaves of magic, it has a 200 turn timer so you can take your time
(3. debug)