I've done a quick search and it seems like there's already a few lists with good spots to EV train your mons in USUM gonna post it here so the people not on Discord can make use of it as well since this list is pretty handy. HP - Lush Jungle, Bounsweet Attack - Route 1, Pikipek Defense - Wela Volcano Park, Cubone Special Attack - Melemele Meadow, Oricorio Special Defense - Melemele Meadow, Flabebe or Brooklet Hill, Dewpider Speed - Diglett's Tunnel, Zubat (Or Route 1, Pichu)
Another great spot I noticed was Malie Garden. Pokémon are between mid to high 20s but can spare you half the time. Ledian and Araquanid both give 2 sp.def evs and total a 40% encounter rate together. Poliwhirl give 2speed evs and Alolan Meowth give 1 (can also SOS call persians that give 2 speed evs) and together they have a 40% encounter as well.
Psyduck at Brooklet Hill are great for grinding Sp. Attack. I found they call for help a lot of the time, and they're quite common.
If you're in late/post-game, Poni Plains has some great grinding opportunities with Fearow, Gumshoos, and Chanseys all possible encounters, and all giving 2 EVs in their respective classes (Fearow 2 Speed, Gumshoos 2 Attack, and Chansey 2 HP, 3 if it calls in a Blissey). Chanseys are also GREAT for SOS EXP grinding. I managed to evolve all the fossilmons and pseudo legendaries from their base forms with this method. Chansey is only found in the rustling bushes, but if you get one, drop that adrenaline orb and watch the XP flood in. I briefly experimented with Oricorios for Special Attack, but their dance moves are annoying as heck to deal with. I recommend going with Elgyems on Hokulani if you want to train up this stat.
