Try the place right of the Pokemon Center in Hearthome City .If she says "That Evee I gave you ,I got it from a good mate from Johto" that means you've already have one.They can also find it in Trophy Garden but it's really hard.
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I'm not sure if you can catch Eevee in Pokemon pearl but you get it as a gift from Bebe in Hearthrome City after you get the national dex.
national dex and a whole lot of luck (only a 5% chance of one showing up)
eevee does only evolve into glaceon in dimond, pearl, platinum ss and hg
It takes AT LEAST 9180 steps to hatch an Eevee.
You can't find Eevee in Pokemon Ruby, but you can trade for it from Fire Red, Leaf Green, and XD.