Depends on the game. These are the only off region starters you can obtain in gens 2-6.
Emerald: One Johto starter for completing the Hoenn dex (you might as well get them from Colosseum, where they're easy to get).
Heart Gold/Soul Silver: One Kanto and one Hoenn starter in the postgame.
X/Y: One Kanto starter from Sycamore early on; you can also find Kanto and (though they're not off-region) Kalos starters in the Friend Safari.
Omega Ruby/Alpha Sapphire: One Johto, one Sinnoh, and one Unova starter in the postgame.
In Sun/Moon, you could get the Johto and Unova starters by Island Scan on specific days, and in USUM, you can get the Kanto, Hoenn, Sinnoh, and Kalos starters by Island Scan.
In SwSh, a Charmander is given to you after beating the game. A Bulbasaur or Squirtle is given to you during the Isle of Armor quest. The Hoenn starters can be found in Dynamax Adventures in the Crown Tundra. The Alolan starters are given to you by the Alolan Diglett hunting man in the Isle of Armor after observing enough Diglett.
In BDSP, Gen 1-4 starters are available in the Grand Underground after obtaining the National Dex.