In Platinum, (I don't know about anything else) you can see each evolution of Eevee. There's a trainer somewhere on the routes between Pastoria, Hearthome, and Veilstone that has an Espeon and an Umbreon. You can also see a Glaceon on the route to Snowpoint, and a Leafeon in between Pastoria and Hearthome. A collector has Jolteon, Flareon, and Vaporeon in between Veilstone and Pastoria. Just search that area and you will find most of the evolutions.
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That would be Umbreon if you're looking for 168 in the Sinnoh Dex. The trainer who has it in their possession is on the south part of Route 212 - his name is Collector Dean. or you could just grab the Eevee from Bebe in Hearthome City and train it and then level it up at night until it evolves into Umbreon. where abouts
If you go onto that muddy trail past pastoria city, you will see a collector trainer, battle him and he has a Umbreon and a Espeon
there is no other way
sorry you cant, you have to evolve your evee during the night it will be either a umbreon or a espeon. so its the matter of luck on getting the one you want.
By trading it in from elsewhere, eevee is not available in the main Diamond/Pearl game before the national pokedex. When you have the national dex, however, you just go to see Bebe in Hearthome, in the house right next to the Pokemon center.