Where can you get vs seeker in HeartGold?

You can't obtain the VS Seeker in Pokémon HeartGold. The VS Seeker was introduced in Pokémon Diamond, Pearl, and Platinum, so it isn't available in older games like HeartGold.

The VS Seeker allows you to re-battle trainers you've already defeated. It's a great way to farm for experience points or rare items.