The collective effort of Project Pokémon to preserve and provide in-game Pokémon data.
Drag the .PKX/.GP1/.CK3/.XK3/.PB7 file into the appropriate save that is opened by PKHeX.
In that case, please browse our gallery here: https://projectpokemon.org/home/files/category/85-in-game-series/
It is pretty much filled with relevant information regarding the respective files.
This is still a work-in-progress, and likely not as complete as our on-site gallery.
While the files ARE legal, as usual, use at your own risk!