I've already caught one, but my sister wants one too, and she'll only trade with me so I can get Milotic if I catch hers (meanie) but I'm not giving her my prism scale :)
Anyway, I've had a giant mental blank. I've tried using my super rod on route 1, but all I get is a bunch of Basculin! >:(
It's rare when using the Super Rod. Be patient. You can improve your chances by fishing in the rippling water.