Celadon City.
You will find him in Mt.Mortar. Pokemon Ace Trainers Ace Trainer Olivia
You can't find registeel in Pokemon HeartGold.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.
unkowns {pokemon}
You cannot find Latios in Pokemon HeartGold unless you went on wi-fi and got the Enigma Stone
You can't
yes
Just find them!
You can't
trade
You can't do it.
There is no such thing.