In Fire Red and Leaf Green, Eevee will only evolve with the Following

Fire Stone - Flareon
Thunder Stone - Jolteon
Water Stone - Vaporeon.

If you are wanting Espeon or Umbreon, You will not be able to get it without the use of trade with another game. The reason is that there is no real time clock in Fire Red and Leaf Green.

Here is a Copy\Paste from Super Creatures

trade eevee to r/s/e, max out its happiness in r/s/e, level it up once in r/s/e after maxing its happiness, then trade it back.

or, trade it from colosseum/XD/any other game that you can find that has one.

eevee cannot evolve into espeon/umbreon in fr/lg due to the lack of a real-time clock.

IN R/S/E, AM=umbreon, PM=espeon.

NOTICE I SAID IN R/S/E, NOT G/S/C.

if you want to say i'm wrong and sprout some BS about day=espeon, night =umbreon, i suggest you go (back) to the g/s/c boards.