If you really want one of the fish, you're better off with Magikarp.
You have an 80% chance of finding one while fishing on Route 2, and it evolves into the decent Gyarados at only level 20.
Feebas on the other hand, you also catch by fishing on Route 2, but on the lakeside area, which you only get access to when you get the water upgrade for the bike you're going to get/may have got already. The water upgrade is received on Route 9. Even then, it's a 1% chance.
Raids containing Feebas are in the second half of the Wild Area, but you need to beat the first 3 gyms to go there, and Milotic itself is only obtainable with that water bike upgrade. Even if you could get a Feebas, to evolve it, you need to trade it while it's holding a Prism Scale.
You're definitely better off with Magikarp.
Hope I helped!