Question

Where can you find a mew in Pokemon heart gold?

Answer 1

It is only available through an event that was over or through a trade with another game or person.

Here are the event Mew details so you know if you found a legit one.

Source

Answer 2

You cannot find mew in Heartgold. You can only find it in emerald. You will have to trade it, out receive it in an event.

Answer 3

You can find a lot of Kanto Pokémon in HeartGold. You can find Mewtwo but not Mew. The only way to obtain Mew in HeartGold is to get in in a Trade, Action Replay, or in the Event that has passed.

I hope I helped you through this post.