Trade it to Pokemon diamond or Pokemon pearl game. Then for leafeon go to eterna forest, you will find mossy rock, level up your eevee around it and your eevee evolve into leafeon; and for glaceon go downside the snow point city to the icy Blizzard, you will find an icy rock rock, level it up and you got glaceon. If you are playing in smartphone then you can only use the cheat of encountering leafeon/glaceon everytime in the wild because trading Pokemon yourself in any DS emulator is almost impossible, even due to the lack of good grass Pokemons I too seriously want leafeon but only cheat can help me for that.
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You can get a Jolteon with a Thunder Stone, a Flareon with a Fire Stone, a Vaporeon with a Water Stone, an Umbreon by getting it to max happinesss during the night, and you can Espeon by getting it to max happiness during the day. You cannot evolve Eevee into Glaceon or Leafeon in HeartGold or SoulSilver. Eevee evolves into Glaceon when you level it up at the Icy Rock at Rt 217 or at the Icy Rock in Twist Mountain. Eevee evolves into Leafeon when you level it up at the Moss Rock at Eterna Forest or Pinwheel Forest. As Rt 217, Twist Mountain, Eterna Forest and Pinwheel Forest are only available in Diamond, Pearl, Platinum, Black and White, you cannot evolve Eevee into Leafeon or Glaceon in HeartGold or SoulSilver.
In Pokémon HeartGold, you can only evolve Eevee 5 ways which would be: Using a Fire Stone on an Eevee into to evolve it into a Flareon, use a Thunderstone on Eevee in order to evolve it into a Jolteon, use a Water Stone on Eevee in order to evolve it into a Vaporeon, evolve it into Umbreon via level-up at night (which is between 8PM and 4AM) when it's happy enough and evolve it into Espeon via level-up during the day time. You can also evolve Eevee into Glaceon and Leafeon however evolving Eevee into Glaceon or Leafeon is not possible in "HeartGold" or "SoulSilver," Eevee can only evolve into Glaceon via level-up when it's near the Ice Rock which can only be found in the Sinnoh region near Snowpoint City and Eevee can only evolve into Leafeon when it's near the Moss Rock which can only be found in the Sinnoh region which is in the Eterna Forest.
If you get it 8 haircuts/massages and then level it up, it should evolve. Galceon, Leafeon, Megezone, and Probepass must be traded for.
Espeon, Umbreon, Glaceon, Leafeon, Flareon, Vaporeon, and Jolteon
Can only evolve from Eevee if it gains a level while on Route 217, so you must trade an Eevee over to Diamond, Pearl, or Platinum and evolve it there, since there is no Route 217 in HeartGold and SoulSilver.