Question

Is there any way to catch a chikorita after you've selected a starter?

Answer 1

Nope.
You can only receive Chikorita as starters.

Comments

How are you so fast, and wheres your source?

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Experience and http://bulbapedia.bulbagarden.net/wiki/Chikorita_(Pok%C3%A9mon)#Game_locations

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Mew is a 24 hour person

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@ pokeZane: You don't need to add a source to EVERYTHING if they knew from playing the game. In this case, Mew already knew. A source isn't needed in this situation.

Answer 2

No you can't. The only way to get it is by the starter that Prof. Elm gives you.