itch.io

Depends on where you are at in the game, EARLY there is a good chansey spot in the bottom floor of the casino tower (It's in the dessert area where the first gym is), (You can also farm lucky eggs for a greater XP boost)

I know of 2 other chansey trainers, one in sparta on a roof top, and another in an end game town's graveyard,
the first one I got the most use out of, I didn't really need to farm xp later on once I got a sturdy team set up.

The paradox pokemon offer good amounts of XP as well, once you get there to help you out.

You can also re-fight all three members of the weather trio, alongside a small hand full of other static legendries over and over again.(Some legendries don't stick around after you knock them out, so make sure to save before hand as a safety precaution.) I got my entire team to level 100 while shiny hunting the weather trio myself

Hope this info helps you out! And anybody else who might be looking for XP farming advice!