In maple story were can you get steely throwing knives

You cannot get steelys from a store, but you can get them most likely at the FM, where stars are most commonly sold. The price is usually over a million mesos. Another way to get steelys are to kill monsters. The monsters that drop steely are :

Leprechaun (45), Night Ghost (60), Panda (60), Red Drake (60), Selkie Jr. (60), Slimy (63), Rash (70), Tauromacis (70), Dark Rash (74), Dark Klock (76), Dual Beetle (76), Yeti and Pepe (78), Haf (80), Blood Haf (83), Dual Ghost Pirate (87), and Elderwraith (95).