I like using Wailmer around Seafolk Village/Poni Wilds for HP EV training, since they sometimes call Wailord who have a higher yield, but that requires getting one below level 29 so it doesn't know Rest, and some way of protecting my guy from Water Pulse's confuse like Substitute. I may just be going for something that's good enough though.
In general I've found that SOS chains are more about what you're wanting to fight and finding a location based on that. You want something that's likely to call for help and doesn't know annoying moves. Pikipek on route 6 all know Supersonic, but on route 1 they only have Peck/Growl. And they're there with Yungoos, which is another 1 attack and has tackle/leer. But anywhere but route 1 Yungoos can carry Sand Attack, you see what I'm getting at?
If you don't mind it being much slower, Isle Evelup at level 3 gives 4 EVs per training session, so you can get whatever spread you want. It does take 31 and a half hours with beans, but you don't have to do anything but check back every now and then.