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Okay, so Feebas can be found on Route 119...fair enough. What you have to realize, though, is that Feebas can't be found EVERYWHERE on Route 119; it is relegated to but six of roughly 400 water tiles on this particular route (FYI, your character stands on one "tile" at any given time and every step he/she takes is one tile length, just to give you an idea of how many spaces you'll have to check). The tiles that Feebas inhabits are completely random; there's no point in asking your friend where THEY caught Feebas because its location will undoubtedly be different in your game (though if you actually know someone who caught Feebas, beating them up and stealing it IS an option...you little Team Rocket Grunt in training, you). In the few tiles that DO contain Feebas you have about a 75% chance of finding one, so fish each tile a few times to make sure you don't miss it (it is recommended that you use the Old Rod for this venture, since if you use the Good or Super Rod you'll be running into a lot of Carvahna). The six Feebas tiles can be randomized by changing the "popular phrase" in Dewford Town, though I wouldn't recommend you do this since you'll be doing a lot of searching either way and if you randomize the tiles you'll have erased all the tiles you've eliminated as potential Feebas haunts up to this point. Conversely, as long as you leave the phrase in Dewford alone the Feebas tiles won't change, so I suggest once you find one you make a note of its EXACT location so you'll never have to search for it again (though making sure you catch at least one female as long as you're there so you can breed it is an even better idea). Personally, I loathe hunting for Feebas...the fact that it's so well hidden is maddening enough, but the way Route 119 is always so rainy and dark is even more depressing for me. Go figure.

this was taken from http://faqs.ign.com/articles/427/427499p1.html 

Step One: Find your self a Feebas (Of course)