Where can I find Feebas?

Thats where he is. It's almost impssible to get one. I advise reading the Feebas Walkthough in the walkthrough section.

He's on route 119, it may be hard to find but it's not impossible,he only appears on 5 tiles out of the total amount and these will change every time you change the saying at dewford, this is what I did.

First I Registered The Old rod,the reason I picked the Old rod was because the only Pokemon you can find with are tentacool, magikarp and feebas(when you find one of the 5 tiles) otherwise you'll get carvanha and other pokemon. If you don't have it talk to the man at the shore of Deword near the cave.

Search every tile it may get annoying but you need him to complete your pokedex.Fish on every tile about 5 or 6 times each more than 6 and you're wasting you're time less than 5 and it could slip through.
Make sure to have lots of pokeballs because your friends might want one.
I caught one on the first tile I checked and I caught about fifteen, but I was just lucky. I tried it on Emerald and it took me about 5 or 10 minutes to catch one. So be patient you'll find one.

P.S
To evolve it to Milotic get his beauty up to 270, it's near full, you need to give him pokeblocks, Blue and Indigo work best.

Hope I helped you and anyone else who reads this:-D

You can get FEEBAS on route 119 but it is not impossible were you have to be patient and use each type of rods I suggest the old rod because it is easy to get him with and also be patient and try to fish patiently to get him.