there are power items which give you a +4 bonus in an ev stat.
It's worth mentioning that Pokerus is applied after the Power item. So say you were battling a Basculin, holding a Power Anklet with PKRS: You'd get (2+4)*2 EVs overall, 2 for the Basculin, 4 added on from the Power Anklet, then PKRS doubles it to a not too shabby 12 EVs for one Pokémon!
But since you don't have PKRS, if you want 252 EVs in a stat, then this is how many battles you need to do:
1. Get 10 of the respective Vitamins and give them to your Pokémon
2. Take them to battle a Pokémon that gives the EVs you want.
If a Pokémon gives 2 EVs then you want to battle once without Power item held and 25 with it and if it gives 1 EV then you need to battle it Twice without and 30 times with
And here's a list of Pokémon that are good for EVing in each stat:
HP: Stunfisk in the surfing area of Iccirus city (where the massive puddle is). 2 HP EVs each. Also, word of advice, use a special ground type if you can, Static and Discharge are annoying! Palpitoad in the puddle also give 2 HP EVs as well, so if you see one on the way, feel free to kill it xD. Also I would recommend doing HP first, because of the levelling formula; it'll often raise your Pokémon enough to start doing the battles by itself.
Attack: Patrat and Lillipup on Route 1. 1 Atk EV each.
Defence: Roggenrola in Wellspring Cave. 1 Def EV each. Preferably take something with Mold Breaker because they all have Sturdy.
Special Attack: Litwick and Elgyem in Celestial Tower. 1 EV each. Take something that resists Burn or doesn't care about it.
Special Defence: Frillish surfing off Driftveil city. 1 EV each.
Speed: Basculin wherever you can find them really, I use the water between Striaton city and the day-care, personally. 2 EVs each.
If you don't have enough money to buy the vitamins, then you can get them in the battle sub for 1 BP each and failing that, just add on 25 battles with the Power item if they give 1 EV or just do 42 battles with the Power item if they give 2 EVs.