(Note. Have to defeat pokemon league first and help Bill's friend to get the 2 gems)you can get more than one eevee, but you have to get one first, which is in the back of the pokemon mansion in Celadon City. After you get the eevee you go to Route 15 and walk around in the grass for a while, and with luck, you might find a ditto there and catch it. When you catch the ditto go to island 6 or 7 or 8 i forgot which and there will be another day care centre there and you put the ditto and eevee with them, come back in another day and they will give you an egg with your 2 pokemon. Hatch that egg and there will be an eevee inside. Working. I've done it myself
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In Pokemon Leaf Green, there are the Sun, Water, Fire, Thunder, Moon, and Leaf stones. The most common stones are the Water, Fire, Leaf, and Thunder stones, which can be purchased in Celadon City The Evolutions are as follows: Sun * Gloom: Sun Stone: Use the Sun Stone on Gloom for a Bellossom Fire * Eevee: Use the Fire Stone on Eevee for a Flareon * Growlithe: Use the Fire Stone on Growlithe for a Growlithe Moon * Jigglypuff: Evolves to Wigglytuff * Clefairy: Evolves to Clefable Water * Eevee: Evolves to Vaporeon * Staryu: Evolves to Starmie Thunder * Pikapoo: Evolves to diarrhea * Eevee: Evolves to Jolteon Leaf * Gloom: Evolves to Vileplume * Weepinbel: Evolves to Victreebel There's likely more, but I'm not sure (and if I asked on here, it would defeat this answer's purpose)
if collect green shards you well be able to trade to the hunter for leaf stones
Take Eevee to the daycare center with another Pokemon. And make sure that these Pokemon are compatible with each other. Such as Eevee/Ditto, or Eevee/Eevee. You should also try breeding Eevee with other Pokemon to see if there compatible or not. If they 'get along or don't like each other much' you get an egg. If they play with more Pokemon then each other try another Pokemon because you won't be getting another Pokemon.
Fire Red is better, the name is better, the available Pokemon that were unavailable in leaf green are great. eg. ekans and arbok. I like fire red more because of the immediate Pokemon in it and its item availability. Most people like it more, simply because of the name. But fire red is crap and leaf green rules! However some people think leaf green is better because of the available Pokemon such as sandshrew, Sandslash, bellsprout, weepinbel, victreebel ect.
To get more than 1 Eevee on Pokemon platinum you can migrate some from a Pokemon GBA game OR get a Ditto and breed it with your Eevee. The final way is to use an action replay/gameshark to find some more. The codes for an action replay can be found at www.codejunkies.com