There are 4 ways if you have a sibling with Chikorita.Without 3
You might not want to do this one though restart your game go to that point and select Chikorita.
Go on wonder trade and try your luck. I got many starters there.
Try GTS many people ask for strong Pokemon though.You could still get one there.
Hopefully you have a sibling which you could trade with.
Good luck getting your chikorita. Oh yeah you could also trade with friends,acquaintances,or passersbys which makes it 4/5.
Source: Knowledge Hope I Helped!:D