actually you can to do so get the evee that's in the game evolve it to jolteon or flareon or vaporeon then put it in the four island day care with a ditto then hatch the egg you get and its evee.
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Breed in the daycare with ditto
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
You can; they're just a bit more rare than the male ones. The first you find in Celadon city is most likely to be male, but you can save just before picking it up and do the process under you get a female one. If you, breed a male Eevee with a Ditto and your egg has a chance of being a female one.
Breed eevee with a ditto. This works for both male and female eevees, although if you are lucky enough to own a female eevee then there are other Pokemon you can breed it with. Check http://www.serebii.net/pokedex-dp/133.shtml for a list
yes they do, but the highest lvl Pokemon you will face is no more than 75 which is your rivals starter Pokemon