You can find FEEBAS in route 119, but,it change place "IN" the route.I suggest you to use your rod 10 times at same place, and , if it not appear,change little bit the place and do the same.
You go to the stream near the weather instutite. Depending on what the dewfod trendy phrase is, Feebas will apppear in different locations. Just keep fishing in each square thrice until you find it. If you keep at it, you will find Feebas in a few hours.