Chikorita is unavailable in-game in XY.
However in ORAS you can get it from Professor Birch after saving him from a Shroomish. If you have Pokebank in a matter of minutes you can get Chikorita from ORAS to XY, if you do not have Pokebank you will need to ask someone to trade over for you.
If you can find a sane offer for a Chikorita on GTS then GTS is a good place. However there are many crazy things running out there and don't be surprised when people want a Diancie or Darkrai or whatnot for a Chikorita.
There are many trade forums available.
NuggetBridge Forums, Reddit, Marriland our Chat Room, Pokemon Showdown WiFi room, Serebii, Smogon Forums. I'm sure you can seal a nice deal somewhere there.
If you have a female Meganium or Bayleef breed it with a male Pokemon in the Monster or plant Egg Group and a brand new, fresh Chikorita will be on it way.
If you have a male Meganium/ Bayleef breed with a Ditto and that way a Chikorita will be born too
Good Luck finding your Chikorita :D