Wrote this document today showing what pokemon give what EVs in every location, did a LOT of surfing on the wiki (so if anyone sees info I got off the wiki which is inaccurate, let me know. In the meantime, hope this helps). Spent all day on this. Hopefully this helps some people with their own EV training or in making a superior “Best EV training locations” guide.
Note that I did not include fishing, hidden grottos, or rock smash encounters due to unreliability and tedium, or 1 and done interactions due to not being repeatable.