How do you get Eevee in Sinnoh?

For Diamond & Pearl, you'll beat the Elite Four and Cynthia then go to Hearthome City. (Not sure if you need to complete the Pokedex and get a Pokeradar.) Go to the house next to the Pokemon Center. (You should save right next to a girl with blonde hair inside the house if you want a female Eevee. Their rare. Eevee's are usually male.) Talk to the blonde, she's Bebe. Bebe will give you an Eevee. For Platinum, you will arrive in Hearthome City. I never really played Platinum before so I'm not sure if you get an Eevee when you arrive in Hearthome City, but if you don't, go to Bebe's and talk to her, she'll give you one and you don't have to beat the Elite Four. Hope I helped!

Christine $ Anabel ;D