I caught a Feebas in my Ruby game. It took me quite a while to find it because it was in the middle of Route 119. I started from one end of Route 119 with my super rod and fished 5 times per tile. I think in total it took me about 3-4 hours that I distributed over a few days. I just made sure that I never changed my trendy phrase and moved tile by tile from one end of the route to the other end. It got pretty boring and frustrating after a while. The good thing is that the tile does not change in R/S/E so that when you encounter one, you just have to remember the tile location and keep fishing at that tile as long as you do not change your trendy phrase. Once I caught my Feebas, I bred a lot of them, so now I have a mini Feebas farm on my Diamond game after I pal parked the Feebas and let it spend some time in the day care with Ditto.
Now it shouldn't be that hard to get a Feebas because there's GTS and forums that help set up Wi-fi trades. Searching for Feebas can be very frustrating. If you look at it as a fun challenge, it isn't that bad, but it can be pretty time-consuming if you aren't lucky. Wasting time and frustration over a measly Feebas isn't worth it. If you want a Feebas, the quick way is GTS and the Wi-fi trade section of this forum. It'll probably be quicker and less stressful.