umbreon
I have them in Heartgold
probably the same way you got it in gold and silver before battle all the girls with eevees evolutions in ecretek city(expect glaceon or leafeon i don't really know) and the man with the rhydon will give it to you after youhave defeated them all.
no, all the Pokemon in black and white are new; there are no evolutions of old Pokemon...
Unfortunately, no, there aren't any other eevees in Pokemon FR/LG, but wait until after you beat the elite four - On Four Island, you can breed your eevee (or any of it's evolutions) with a ditto, and you'll have plenty of eevees.
That's easy. just catch a geodude. ( or one of geodudes' evolutions .)
Eevees
put it in day care with another normal type and hatch an egg(evee has to be female)
Eevee is breeded by three ways. The first is if you have two eevees (Boy and Girl). The second is use two of eevees evolutions. The last is use one eevee and breed it with a ditto. Hope this helps!
Lvl 20. I recomment serebii.net for Pokémon evolutions and where to find them. Just search your pokémon + serebii.
There is no best Pokemon on Pokemon HeartGold.
The best way to get all the evolutions of Eevee are to 1: breed the initial Eevee until you have the required amount; 2: use the stones related to evolution or level them up during a certain time of the day. You can look for evolution faqs on google if you want more specifics. As long as you breed Eevee you can have an infinite amount.
Girl Eevees are rare. Because more than 95% of the time boy Eevees hatch instead of Girls. Out of my overall 60 Eevees, 55 are boys. But it is possible.