I answered this question already.
Here are the best places to EV Train in FireRed/LeafGreen. Please note that other Pok�mon may pop up in these areas. If they are not listed, run away from them so you don't get EVs in the wrong stat - it shouldn't happen too often.
HP - Wooper (FR), Marill (LG) - Ruin Valley Water - Wooper gives 1 EV while Marill gives 2 EVs.
Attack - Paras - Mt. Moon B1F - Paras gives you 1 EV in Attack.
Defense - Tangela - Route 21 - Nothing else pops up here and Tangela gives you 1 EV in Defense.
Sp Atk - Gastly, Haunter - Pok�mon Tower - Gastly gives 1 EV, Haunter gives 2 EVs in Special Attack.
Sp Def - Tentacool, Tentacruel - Kindle Road Water - Tentacool gives 1 EV, Tentacruel gives you 2 EVs in Special Defense.
Speed - Diglett, Dugtrio - Diglett's Cave - Nothing but these babies in here. 1 EV and 2 EVs in Speed, respectively.
ALWAYS KEEP TRACK OF YOUR EV TRAINING. That is so important, I can't stress it enough.
Fight nothing, but the desired Pokemon for the wanted EV's and aways keep track on how you EV train it. If you don't you will forget what you did.
Say you fight 4 Geodude each you beat is worth 1 EV defense point. That means you gained 4 EV's on defense. The max each Pokemon stat can reach is 255. The max EV points you have to spread around your Pokemon's stats is 510. If you give your Pokemon 252 Attack EV's, and 252 Speed EV's. 255+255=510. You will have no more EV's to spread around your Pokemon's stats so think carfully, and KEEP TRACK.
Yawn - what is there to say?