available in olivine city for 75k. with this rod you can fish certain pokemon, like dratini in dragons den/safari zone
Indeed you need 50k for that quest, however fly doesn't work outside of battle. But you'll unlock the subway system which works like fly, but it also costs 2,5k per travel. (you also need to have the certain leagues defeated in johto)
What I'd suggest as a player: get money for membership -> get bike -> get super rod -> get fly