I don't think there is a DS game with a starter Pokemon squirtle, but you can get one by either going to the GTS (global trading station) and finding one, or just put a GBA game into the GBA slot at the bottom of the DS.
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You cannot get Mewtwo as a starter Pokemon. The only starter Pokemon available are Charmander, Squirtle, and Bulbasaur. There is no other way to get another starter Pokemon. Mewtwo can only be obtained at the end of the game.
Squirtle is not available in Pokemon Crystal. Squirtle can only be found in Pokemon Red, Blue, or Yellow. Squirtle can be traded to Crystal from those titles.
You can only get Squirtle in LGFR.
Simply you can't.Since you've start the game and you've choose a starter Pokemon you can only start a new game or trade with your friends. www.pokemaniacs.forumotion.net
Only by trading a Squirtle to HeartGold.