You can use Vitamins (Calcium and Carbos for Atk and Spe) for the first 100 EVs without giving it any experience. After that, Patrat and Lillipup on Route 1 both give 1 Atk EV, and Woobat in Wellspring Cave has a 50% encounter rate and gives 1 Spe EV. As sumwun said, you’ll also want to use the Power Items (Power Bracer for Atk and Power Anklet for Spe), which give an additional 4 EVs in their respective stat per KO. And if possible, you’ll also want to infect your Pokémon with Pokerus, which doubles all EVs gained (giving you 10 EVs per KO with the Power Items). I would also recommend splitting the EXP gain between at least one other Pokémon, but the more, the merrier.