YES in blackhorn city you go to the old mand's house to trade him a ragecandy for a poke radar
The pokeradar is in the Pokewalker. You can't actually get it in the game.
You can't get pokeradar in Heart gold or Soul Silver.
Yes, someone in violet city I believe, not entirely sure, he will give you the dowsing machine which is the pokeradar, and it will allow you to find items