To get Magmar to evolve you have to trade it while it is holding the Magmarizer item.
you need to swap it with a Friend while holding magmarizerby shaw foxall -
In HeartGold, you can catch level 15-17 Magmar in the Safari Park near Clanwood City.
level 19
36
at level 30
It eveloves at level 42
Cinnabar Island
Magby evolves by leveling and it'll start to evolve into Magmar at Level 30.
Drowzee evelove at level 26
level it up to level 33
Nowhere, you have to breed magmar to get it.
To catch Magmar you have to go to the "Burned Tower". Walk a lot and you will find it.