Answer 1

You can find a Feebas by using your Super Rod on Route 1. They're quite rare, though.

Comments

Is it like in d/p/pt with only one spot?

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No....
You can find them on any body of water, just bring lower level pokemon, as the Feebas are low levels. (Answering Sanne)

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use a max repel, and surf around on route 1 for a while and when the bubbling spot appears it is most likely that it will be a feebass but there is also a small chance of it being a milotic... Hope this helped