tidus gets his overdrives by using them wakka gets his as a prize from blitzball kimari learns them by using lancet on some of the enimies rikku starts of with all of hers so you just need to find the right combination.lulu gets hers by learning new black magic spells auron gets his by finding auron jecht and braska spheres (like the one in macalnia after fighting spherimorph.)yuna gets hers by getting new aeons and i think thats everyone apart from seymour who already has his and also valefor has a second one that can be learned in besaid.