Mantine are quite uncommon but appear on the sea-route between the Whirl Islands and Cianwood. I would take a few ultra balls and dive balls, as these dont like to be caught
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You cannot get mantine in firered only emerald and colosseum can.
In HeartGold, give a Mantine the Wave Incense and breed it. The egg with hatch into a Mantyke.
You can catch Lombre in Pokemon HeartGold in the Safari Zone at a waterside area once you receive the National Pokedex.
Find and then catch
you do not catch it . you can bulbasaur when you get it from proff oak