Mossdeep City.
Old rod, Good rod and Super rod
The super rod in pokemon ruby, sapphire and emerald is in mossdeep city.
Super Rod at any body of water
you have to fish using the super rod or go underwater and look for him
use super rod in meteor falls
You can use either the good rod or super rod the super rod will work more often for finding staryu.
By capturing all of the water types of it.
Use a super rod in the safari zone and hope for a seaking.
fish in the water with a super rod outside of victory cave
By using a super rod in slateport It's simple,use a super rod and go fising at the sea.You can get the super rod in Mossdeep city,in a house next to the space center talk to the fisherman and he will give you a super rod
Depending on where you use it you can find various pokemon like Carvanha, Corphish, Sharpedo, Feebas, Staryu.
use the super rod near the weather center near fortree city in the water.