Gen 1 Scientist in lab on isolated island seeks rare fossils to clone ancient Pokemon.

Gen 2 No way to revive fossils.

Gen 3 Large corporation that creates new pokeballs etc also can clone fossil pokemon.

Gen 4, 5 Cloned by shockingly well funded museum's staff.

Gen 6 Another research lab that clones fossil pokemon.

Gen 7 MAN IN PARKED RV ACROSS FROM MOTEL WEARING FARMER'S CLOTHING TURNS FOSSILS INTO POKEMON.