Where to catch feebas in Pokemon diamon and pearl?

give it alot of dry poffins

give alot of dry puffins to max out its beauty stat.

fly to celestic town go in the cave till you find the floor with the fog and start fishing.

There are onyl ffour spots where you can get him but they are ranom every game. as well there is only about 50% chance of seeing him so try each spot a few times.