Where can you obtain Corphish in Pokemon Sapphire?

Fish for it in Petalburg City using Good or Super rood.

(Try it yourself)

you can also get it outside of odale town in the little pond. they are very popular in there with the super rod.

You can also get one in Mauville fishing in with the Super Rod in the little Pond at the bottom. the one that is near the day care center. I found one right away.