Besides one-time side missions and random pickups on the ground, there are two berry stalls that are a reliable source of these berries.
Magenta Sector 1
On the street corner to the east of Cafe Gallant, a woman is selling all 6 EV-reducing berries.
Magenta Sector 6
There is a marketplace surrounded by buildings in the middle of this sector, with an entrance on the east side that isn't visible on the map. The brown-haired man is selling all 6 EV-reducing berries.
If you want to reset all the EVs on a Pokemon at once, you can request this service from Lady Clear at the Rust Syndicate Office for a price of 5 Mega Shards. Once unlocked through story progression, she is located at the counter on the right wall of the entrance lobby.
Source: Game experience, screenshots are my own.
