Sorties are a sequence of three high level missions (100), the final giving a reward. The reward will rarely be a lens or greater lens, but it is possible. Otherwise obtaining them is done by trading with other players or you can buy greater lens for 40 plat.
Okay, sense some smartassery here, so to get the ridiculous bow known as the "lenz" you need to go to your clan dojo, and buy the blueprint from there if your clan has it researched already. If not, get to work, slacker.