Identification
Magnemite is one of the few Pokemon to appear in the first five Pokemon game generations. He appears as a large metallic orb with an eye in the center. Two magnets are attached to either side of the orb; these give this Pokemon the power of flight. Magnemite was an Electric-type Pokemon that later became an Electric/Steel type when the Steel type was introduced.
Location
Magnemite resides exclusively in New Mauville in "Pokemon Sapphire." This may be because of the large power plant located there -- Magnemites are supposedly drawn to large outputs of electrical energy. New Mauville is located beneath Mauville and provides electrical power to the town.
Getting to New Mauville
Play through the game until you defeat Norman in Petalburg City, and then travel to the intersection between Route 110 and Route 118. Speak to Wattson to receive the Basement Key. Take the Basement Key to Mauville and open the door to the underground. This will allow you to access New Mauville.
How to Catch Mangemite
There are no special tricks needed to catch Magnemite. Walk around until you are attacked by the Pokemon, and then wear it down so it has less than a quarter of its health. Throw a Pokeball at it to capture it. Ground-type attacks are especially effective against Electric Pokemon like Magnemite, but make sure that you don't cause so much damage that it faints. This will prevent you from catching it in that battle.