FindingFeebas is renowned for being an incredibly difficult Pokemon to find. Since it's 'discovery' in Pokemon Ruby and Sapphire, the title of owning a Feebas has graced few and caused many to erupt into tumultuous fits of rage. Finding Feebas can take hours of perseverance, but it is worth it in the end.
Ruby, Sapphire and Emerald: Head to Fortree City and head west down to the banks of the river. Use Surf. Feebas is programmed to appear in six different spots that change everyday. There are approximately 400 tiles, so be prepared to spend a while looking around. Make sure you have some PokeBalls and start fishing in each tile up to three times using any rod. I suggest Old, because Super and Good will give you Carvanha and they can sometimes be a hassle when they don't let you run.
Keep fishing and try to remain calm. (Haha some people just storm off after about thirty tiles) Make sure to check both tiles near the banks, close to the waterfall, above the waterfall and in the smaller seclusion of the river at the southern tip of Route 119. Do not give up.
Be sure to fish in every possible tile. Go up and down the river, or side to side. Any method works but remember to cover all ground (err..water.)
If you're not in luck, you can randomize the tiles again by going to Dewford and changing the trendy phrase. This randomizes the game's random number generator, but doesn't provide you with a set number. So if the phrase is "Cool Blaziken" it won't always be the same tiles in every game.
Diamond and Pearl: Feebas can be found in Mt. Coronet's Underground Lake. They are even HARDER to catch, because they appear in only four tiles now. However, this lake larger, with about 532 tiles but it's also more square, making it easier to keep track of your movements. Scan the lake, moving in rows in an organized direction. Fish a few times per tile, then move on. The tiles randomize per day. You must use a Super Rod.
Platinum: Same as Diamond and Pearl, but you can use any rod to catch them.
Black and White: In Black and White, Feebas is much easier to find. It's actually refreshing how much easier it is. Go to Route 1, north of Nuvema Town and go to the river. Fish around. You'll find mostly Basculin, but you'll eventually stumble upon a Feebas. You can also fish in Rippling Water tiles for an even higher chance. No tiles, no frustration, but you're missing out on half the experience!
CapturingFeebas has a catch rate of 255, making it one of the easiest Pokemon to catch. Any old PokeBall will do. It's not very strong, so it won't put up much of a fight, but it's weak. Do not use any form of attack unless its False Swipe, Super Fang, or something that will not kill it. Try to paralyze or put it to sleep, but do not kill it.
You should have a problem catch it. Congrats! You own a Feebas! Be proud! :D
Evolving ItFeebas evolves after increasing its Beauty, then leveling it up. There are several ways of approaching this, all of which requires more time and patience.
Ruby, Sapphire, Emerald: The way of increasing Beauty in these games is obviously PokeBlocks. You can make these at any contest hall (Lilycove City Contest Hall) by making PokeBlocks with predominately dry berries. Make sure to check each berry for dryness (they're usually blue, but not always) before adding them to the PokeBlock.
I recommend doing this with a friend, since you can control the berries added.
Berries that are dry include: Chesto, Wiki, Cornn, Bluk, Kelpsy, Pamtre and Ganlon.
Now, you can use a thousand Chesto Berries and make minimal-beauty PokeBlocks, or you can be more serious and use Pamtre Berries. These are rare, but you can obtain them by going to the Berry Master's wife (she lives in the house on Route 123 below where the route in which you found Feebas) and telling her the phrase Challenge Contest. She'll give you a Pamtre Berry.
Blend the berries and feed Feebas these PokeBlocks. Watch its Beauty get larger, and when it no longer grows, level it up.
Blue PokeBlocks are best, but Indigo also works as well. Gold and White can work too, if you have the right berries.
Diamond, Pearl, Platinum: Feebas can be leveled up congruently in DPPt through Poffins. Go to the Poffin House in Hearthome City and make some Poffins using Dry berries, like Chesto, Passho, etc. Feed these Poffins to Feebas and level it up.
Black and White: Trade Feebas with a Clean Scale. Kind of defeats the purpose of all the work that goes into Feebas, but meh, it makes it a world easier. xD
Congratulations!You are now the proud owner of Milotic. Make sure to breed it, so you have more. It'd be best to catch a female Feebas, so you don't have to go through the hassle of finding a Ditto, but males work too. Feebas gets some cool egg moves like Haze, Hypnosis and Confuse Ray too, so take that into consideration. :O