You will need to find one in the Safari Zone, or by headbutting trees in the Viridian Forest
In the great marsh. I think you have to see it in the Binoculars first.
you can catch shroomish at the pastoria great marsh between the levels of 27 - 31
Shroomish is a rare Pokemon in Ruby and Saphire but you can find it in the Petalburg woods.
Probably where you catch kyogre. (i don't know i used a repel.)
Headbutt a tree in the Viridian Forest, you might also get a shroomish.
Safari zone, or head butt trees (after the league) to get a shroomish.
You can't catch Rotom in heartgold.
you cant catch him in heartgold you can only get him in soulsilver
In the woods where you go to rustboro, but u first have to catch a shroomish and evolve it
The Great Marsh. Check every day.
You can't catch him in HeartGold. Only in SoulSilver. In HeartGold, you catch Kyogre in Embedded Tower. If you transfer either Kyogre or Groudon to the other game, you can catch Rayquayza
You can catch Ekans in Pokémon HeartGold in the Safari Zone in the Marshlands area.