this is the hardest thing to do in the game...
firstly feebas is found in a pond in mt coronet with a lot of fog there, you also have to surf on a Pokemon and fish with any rod but a super rod really is recommended. plus, feebas only appears on 4 spots in the whole huge pond and the spots change every day... with so much searching roughly 1 month will get you the feebas and when you catch it, catch plenty on the same spot to have a stockin case the evolution part (feeding high quality dry poffins) goes wrong. a feebas with a modest or bold nature that likes dry food is best. get ready to be stressed out by the annoying constant searching.
Feebas
i think national is Omanyte and sinnoh is Feebas
theres a underground lake deep in mt.cornet
you can't just find it you have to evolve a feebas signed:hartm017
you would use a good rod, but its hard to get feebas its location changes in Mt. Coronet
mt.coronet
on the left of sunnyshore city. a fisherman has feebas
mt coronet
fisher man
Feebas
mt corenet
feebas
its in a lake in mt.cornet, its where all the fog is.
theres a hidden lake in mt.cornet
it is found in mount coronet
someone on route 222
the foggy bit in Mt.Cornet