Fortunately for EV trainers, Marriland has a great guide to EV training in each game. Here is what I took from it, specifically on Ruby and Sapphire:
Here is where to EV train for each stat.
- HP: In Rusturf tunnel, Whismur is a 100% encounter and gives 1 HP EV.
- ATK: On Routes 102, 117 and in Petalburg city Corphish is a 100% encounter with the super rod, and gives 1 ATK EV.
- DEF: Underwater on Routes 124 and 126 Clamperl is a 65% encounter and gives 1 DEF EV.
- SPA: On Route 112, Numel is a 75% encounter and give 1 SPA EV.
- SPD: In the Abandoned Ship, Tentacool is a 99% encounter and gives 1 SPD EV. The other 1% encounter is Tentacruel, which gives 2 SPD EVs.
- SPE: Surfing on Route 104 has a 95% chance to yield a Wingull, which gives 1 SPE EV.
Source: Marriland's amazing guide, and EV training my entire team in Emerald.
Remember to use a Macho Brace (From the Winstrate Family) and catch Pokerus. Happy EV training! Hope I helped.