Its in one of the Island pokedexs but db said that you have to trade it from another game so now does that work?
Goomy is able to be found in the Lush Jungle (which Mallow's trial takes places) and route 17. Despite this it can only be caught after been called upon from another Pokemon via SOS battle. What makes it particularly frustrating is that is has to be raining before you encounter a Pokemon. If you find it to tedious or don't have enough time to find a goomy this way I would suggest doing what I did and have a friend trade it to you.