Feebas is a special evolve thats both difficult to catch and quite difficult to evolve. Feebas needs to feel as beautiful as ever then have its level risin. This video will show you exactly how to evolve your Feebas into a Milotic.
The Micle Berry (which can only be obtained from Shaymin from the Toys R Us event) is THE best berry to get dry poffins with. Of coarse you can always plant them in the ground to make more (Usually 2-4 berries per plant). They may take a couple days to grow (don't bother setting the time ahead on your DS because it doesn't work, I've tried). The other berry I recommend is the Pamtre Berry. This berry can ONLY be found in the Amity Park, the place in Hearthome City where you can take specific Pokémon and walk around with them. Set your Pokétch to the Step Counter app. and approximately every 200 steps, your Pokémon will be holding either a Berry or a Fashion Item. Keep trying this, and try with different Pokémon if you don't see a change in the items the Pokémon is picking up. Those are the only two options I have as far as that goes. You'd probably need to make Poffins with other people... if you plan on using "weaker" berries. Also: Make sure your Feebas likes dry food, or else evolving it won't work as well, trust me. Try to make sure when making your Poffins that you are quick at stirring the mix, and not to spill or burn. Don't forget to level up your Feebas once it's beauty is high enough. If your Feebas DOES NOT evolve, then I'm sorry, but you did not get it's beauty high enough. =( PS All those Feebas were legitly caught, and was frustrating lol. To find out how to catch Feebas on your own, just look it up here on youtube like I did lol.